What is the acceleration of the car when ?x = 13.7 m/s ?

Question

A race car starts from rest and travels east along a straight and level track. For the first 5.0 s of the car's motion, the eastward component of the car's velocity is given by υx(t)=(0.820m/s3)t2.What is the acceleration of the car when υx = 13.7 m/s ? 

Answer 1

a=dv/dt=2*0.82*t

what time velocity is 13.7 solve 13.7=0.82*t^2   

t=sqrt(13.7/0.82)  =4.51 s

Use the time in the acceleration expression a= 2*0.82*4.51=7.39m/s^2