Please help me find the answer to part b.

A car suspension can be modeled as a system with two axles connected by a solid bar of length L = L1 + L2. It has a moment of inertia | about its center of mass C, which is a distance L1 from the front of the car. The car has front and back suspension springs with Hooke's constants k1 and k2, respectively. When the car is in motion, let x(t) represent the vertical displacement of the center of mass of the car from equilibrium and let 6(t) represent its angular displacement (in radians) from the horizontal. The equations of motion given below are taken as a model for an undamped car with m = 100 slug, I: 900 ft- lb - s2, L1 = L2 = 5 ft, and k1 = k2 = 1800 lb / ft. Complete parts (a) and (b) below. mx\" = (k1+k2)x+(k1L1k2L2)6 l9\": (k1L1 k2L2)x (k1L+ k2L)e (a) Find the two natural frequencies of oscillation (in hertz). The frequencies are 0.955, 1.592 hertz. (Round to three decimal places as needed. Use a comma to separate answers as needed.) (b) Assume that this car is driven along a sinusoidal washboard surface with a wavelength of 50 . Find the two critical speeds. The speeds are feet per second. (Round to one decimal place as needed. Use a comma to separate answers as needed.)