What is the acceleration of the car when vx = 16.9 m/s?

There is no Vx (t). So let us assume it is = (0.910 m/s^3) t^2.

Since a = dv/dt,

a (t) = 0.91*2t = 1.82t

To look for the t at which v = 16.9 m/s

16.9 = 0.91*t^2

t = sqrt (16.9/0.91) = 4.31 s

a (4.31) = 1.82*4.31

=~7.84 [m/s^2] would be the answer.