A car accelerates at a constant rate from zero to 29.3 m/s in 10 seconds and then slows to 20 m/s in 5 seconds. What is its average acceleration to the nearest tenth of a m/s^2 during the 15 seconds?

Answer is 1.3....not sure how to get to it.

I am using the formula

a= (V sub2 - V sub1) / (T sub2 - T sub1)