How am I doing this problem wrong

Question

On a strange, airless planet, a ball is thrown downward from a height of 17 m. The ball initally travels at 15 m/s. If the ball hits the ground in 1 s, what is this planet's gravitational acceleration?

by definition of acceleration
a = (v - vo)/t
a = (-15 m/s)/(1 s)
a = - 15 m/s^2

sense when it hits the ground its velocity would be zero

Answer 1

-17=-15t-1/2 a t^2
one second, then a= -2*2 m/s^2 check that.

I have no idea what your thinking is on the solution. The velocity is most certainly not zero when it hits the ground.