Answer
$64.8\space km/h$
Work Step by Step
When the suspensions are driven at their natural frequency their amplitude becomes maximum at that time the car begins to shake violently. The suspensions were driven by the bumps on the road. So we can write,
$V=f\lambda $; Where, V - speed of the car, f - driven frequency, $\lambda$ - Wave length (Distance between two bumps)
Let's plug known values into this equation.
$V=0.45\space s^{-1}\times40\space m=18\space m/s$
Let's convert this value into $km/h$
$V=\frac{18\space m}{s}\times\frac{3600\space s}{h}\times\frac{km}{1000\space m}=64.8\space km/h$
