Question

A projectile is fired straight up. y"=-GM/((Y+R)^2) -Be^-y
y(0)=0
y'(0)=V_0

Compute total amount of time for the projectile to stay in the sky from launch till retouch of the ground. G, M, R, V_0, and B are all constants.

y"=a=d^2x/dt^2, but where do i go from there?