A car with a total mass of 1800kg (including passengers) is driving down a washboard road with bumps spaced 5.0m apart. The ride is roughest—that is, the car bounces up and down with the maximum amplitude—when the car is traveling at 6.1 m/s.What is the spring constant of the car’s springs?

Answer 1

what is the period when the resonance occurs?
T = distance/speed
= 5 / 6.1 m
seconds
so frequency f = 1/T = 6.1/5

Now do your sqrt K/m thing