The height h after t seconds of a falling object starting at t=0 at height x feet is h(t)=x−16t^2 (feet).

Let t_1 be the time it takes the object to fall x feet. The kinetic energy of a ball of mass m dropped vertically x feet is E_1=(1/2)m(v_1)^2 , where v_1=h′(t_1). Find the formula for E_1 in terms of m and x.

I've tried substituting values to only get in terms of m and x but am having trouble. Please help, thank you.