> 
> A ball is thrown upward from roof of 32 foot building with velocity of $112$ ft/sec. The height after $t$ seconds is: $s(t)=32+112t-16t^2$.  
> 
>  (a) Find the maximum height that the ball reaches. (answer: $228$)  
> 
>  (b) Find the velocity of the ball when it hits the ground. (answer: $-120.79735$) 
> 
> 
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(a) I took the derivative of the height function to get the velocity function and set it equal to zero, since the maximum height will be at the top of the inverted parabola, and at point the velocity (derivative) is zero (right?): 

$v(t)=112-16t=0 \implies t=7$, then I substituted this $t$ into $s(t)$ to get $32$, which is wrong. 

(b) the velocity of the ball when it hits the ground is when height=$0$, right? I set $s(t)=30$? How do I solve this? 

Thank you.