First of all, you need to know the right formula for the answer.
The formula to calculate the average acceleration is:
Average acceleration = Change in speed / Change in time
First of all, we need to transform these 5 kilometers into meters, and with that, we have that the missile moves from the 0s point to the 10s point with an average speed of 5000m/s.
So, as you know, he starts with no speed and from 0s in time, the formula will be like this:
5000m/s-0m/s / 10s-0s
With this, the final result is 500m/s^2.