diff --git a/semester3/pset7/7-3-Slack-Time.c b/semester3/pset7/7-3-Slack-Time.c
new file mode 100644
index 0000000..b90b0c9
--- /dev/null
+++ b/semester3/pset7/7-3-Slack-Time.c
@@ -0,0 +1,148 @@
+#include <stdio.h>
+#include <stdlib.h>
+#define SIZE 8
+#define INF 99999
+
+typedef struct queue {
+  int front;
+  int rear;
+  int *arr;
+} queue;
+
+queue *newQueue() {
+  queue *q = malloc(sizeof(queue));
+  q->arr = malloc(sizeof(int) * SIZE * 30); // make this larger
+  q->front = 0;
+  q->rear = 0;
+
+  return q;
+}
+
+void enqueue(queue *q, int i) { q->arr[q->front++] = i; }
+
+int deque(queue *q) { return q->arr[q->rear++]; }
+
+int is_empty(queue* q) {
+  return (q->front == q->rear);
+}
+
+int min(int l, int r) { return (l > r) ? r : l; }
+
+int max(int l, int r) { return (l > r) ? l : r; }
+
+int **newMatrix() {
+  int **mat = malloc(sizeof(int *) * SIZE);
+  for (int i = 0; i < SIZE; i++) {
+    mat[i] = malloc(sizeof(int) * SIZE);
+    for (int j = 0; j < SIZE; j++) {
+      mat[i][j] = INF; // make sure every point is not reachable by default
+    }
+  }
+
+  mat[1][2] = 3;
+  mat[1][3] = 3;
+  mat[1][4] = 6;
+
+  mat[2][4] = 2;
+  mat[2][5] = 5;
+
+  mat[3][4] = 3;
+  mat[3][6] = 3;
+
+  mat[4][5] = 2;
+  mat[4][6] = 2;
+  mat[4][7] = 5;
+
+  mat[5][7] = 3;
+
+  mat[6][7] = 4;
+
+  return mat;
+}
+
+void getec(int *ec, int **mat) {
+  int cur;
+  queue *q = newQueue();
+  for (int i = 1; i < SIZE; i++) {
+    // eneueue anything that has in-degree == 0
+    int is_start = 1;
+
+    for (int j = 1; j < SIZE; j++) {
+      if (mat[j][i] != INF) {
+        is_start = 0;
+        break;
+      }
+    }
+
+    if (is_start == 1) {
+      ec[i] = 0;
+      enqueue(q, i);
+    }
+  }
+
+  while (!is_empty(q)) {
+    cur = deque(q);
+    for (int j = 1; j < SIZE; j++) {
+      if (mat[cur][j] != INF) {
+        ec[j] = max(ec[j], ec[cur] + mat[cur][j]);
+        enqueue(q, j);
+      }
+    }
+  }
+}
+
+void getlc(int *lc, int* ec, int **mat) {
+  int cur;
+  queue *q = newQueue();
+  for (int i = 1; i < SIZE; i++) {
+    // enqueue any point that is an end
+    int is_end = 1;
+    for (int j = 1; j < SIZE; j++) {
+      if (mat[i][j] != INF) {
+	is_end = 0;
+	break;
+      }
+    }
+
+    if (is_end == 1) {
+      lc[i] = ec[i];
+      enqueue(q, i);
+    }
+  }
+
+  while (!is_empty(q)) {
+    cur = deque(q);
+    for (int j = 1; j < SIZE; j++) {
+      if (mat[j][cur] != INF) {
+        lc[j] = min(lc[j], lc[cur] - mat[j][cur]);
+        enqueue(q, j);
+      }
+    }
+  }
+}
+
+int main(void) {
+  int **mat = newMatrix();
+  int start, end, node;
+  scanf("%d,%d,%d,", &start, &end, &node);
+
+  mat[start][end] = INF;
+
+  int ec[SIZE] = {};
+  int lc[SIZE];
+  for (int i = 1; i < SIZE; i++) {
+    lc[i] = INF;
+  }
+
+  // start from 1
+  getec(ec, mat);
+  lc[SIZE - 1] = ec[SIZE - 1];
+  getlc(lc, ec, mat);
+
+  printf("%d", lc[node] - ec[node]);
+  // for (int i = 1; i < SIZE; i++) {
+  //   printf("%d: %d - %d = %d\n", i, lc[i], ec[i], lc[i] - ec[i]);
+  // }
+
+  return 0;
+}