A child's toy consists of a piece of plastic attached to a spring. The spring is compressed against the floor a distance of 2.00 cm, and the toy is released. If the toy has a mass of 90 g and rises to a maximum height of 50.0 cm, estimate the force constant of the spring (in N/m).
I need help on which equation/formula to use and how to apply it.
Potential energy of the spring PE = 1/2 k x^2