A cars velocity changes from 20ms to 50ms 1o seconds later what is the cars average acceleration?

A cars velocity changes from 20ms to 50ms 1o seconds later what is the cars average acceleration?
To find out an average for anything, you need to take 20 and 50, multiply the two numbers, divide by how many numbers there is, which there is only 2, so it averages to 50ms. I am still in school so I know this is the correct answer