A ball is thrown vertically upward from the top o f a 100-ft-high building. Its height above the ground at time t is given by h(t)=100+30t-16t^2

At what time is the ball falling toward the earth at 46 ft/sec?

I've worked this problem several times, yet I always come out with the answer as t=-1/2. The answer is 19/8, according to the book.

I take the derivative of the equation, since I'm dealing with velocity, and come out with v(t)=30-32t.

Then I plug 46 in for the velocity, since the unit can only be used for velocity, and I solve.

I know I'm not doing it right, but where am I going wrong?

Please help! Thank you!