a rock is thrown straight upward with an initial velocity of 50 m/s from a point 100 m above the ground. If the acceleration of the rock at any time t is a= -10 m/$\displaystyle s^2$ , what is the maximum height of the rock (in meters)?

here's my attempt:

a(t)= -10t

take anti-derivative

$\displaystyle v(t)= -5t^2+c$

v(0)= -5(0)^2+c

50= c

$\displaystyle v(t)= -5t^2+50$

$\displaystyle s(t)= \frac{-5}{3}t^3+50t+100$

then i dont know what to do from here