When the ball hits the ground, the height is 0.
0 = 100 - 16t²
0 = (10 - 4t)(10 + 4t)
0 = 10 - 4t or 0 = 10 + 4t
4t = 10 or -4t = 10
t =
or t = 
Time cannot be negative (unless you have a time machine) so disregard 
Answer: t =
= 2.5 seconds
The answer to number 1 = 40
Take the root of both sides and solve.
c = - 7 sqr root 5
c = -1.258
I would recommend calculatorsoup.com for the fractions part.