diff --git a/semester3/exercises/exercise3.21.c b/semester3/exercises/exercise3.21.c
new file mode 100644
index 0000000..1652654
--- /dev/null
+++ b/semester3/exercises/exercise3.21.c
@@ -0,0 +1,105 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct doubleStack {
+  int capacity;
+  int top1;
+  int top2;
+  int *arr;
+} doubleStack;
+
+doubleStack *createStack(int capacity) {
+  doubleStack *new = malloc(sizeof(doubleStack));
+  new->capacity = capacity;
+  new->top1 = -1;
+  new->top2 = capacity;
+
+  if (capacity <= 0) {
+    return NULL;
+  }
+  new->arr = malloc(sizeof(int) * capacity);
+
+  return new;
+}
+
+void removeStack(doubleStack *st) {
+  free(st->arr);
+  free(st);
+}
+
+void push(doubleStack *st, int in, int side) {
+  // side == 0: top1
+  // side == 1: top2
+  if (st->top1 == st->top2 - 1) {
+    printf("ERR: array is full\n");
+    return;
+  }
+
+  if (side == 0) {
+    st->top1++;
+    st->arr[st->top1] = in;
+  } else {
+    st->top2--;
+    st->arr[st->top2] = in;
+  }
+}
+
+int top(doubleStack *st, int side) {
+  if (side == 0) {
+    return st->arr[st->top1];
+  } else {
+    return st->arr[st->top2];
+  }
+}
+
+int isEmpty(doubleStack *st, int side) {
+  if (side == 0) {
+    return (st->top1 == -1);
+  } else {
+    return (st->top2 == st->capacity);
+  }
+}
+
+void pop(doubleStack *st, int side) {
+  if (!isEmpty(st, side)) {
+    if (side == 0) {
+      st->top1--;
+    } else {
+      st->top2--;
+    }
+  } else {
+    printf("ERR: array is empty\n");
+  }
+}
+
+int main(void) {
+  doubleStack* test = createStack(5);
+
+  push(test, 1, 0);
+  printf("Top of left stack is %d\n", top(test, 0));
+  push(test, 2, 0);
+  printf("Top of left stack is %d\n", top(test, 0));
+  push(test, 3, 0);
+  printf("Top of left stack is %d\n", top(test, 0));
+  push(test, 4, 0);
+  printf("Top of left stack is %d\n", top(test, 0));
+  push(test, 5, 0);
+  printf("Top of left stack is %d\n", top(test, 0));
+  push(test, 6, 0);
+  printf("Top of left stack is %d\n", top(test, 0));
+  pop(test, 0);
+  push(test, 6, 0);
+  printf("Top of left stack is %d\n", top(test, 0));
+
+  pop(test, 0);
+  pop(test, 0);
+
+  push(test, 9, 1);
+  printf("Top of right stack is %d\n", top(test, 1));
+  push(test, 8, 1);
+  printf("Top of right stack is %d\n", top(test, 1));
+
+  removeStack(test);
+
+  return 0;
+}