-100=0*t+0.5*-9.8*t^2

Solution for -100=0*t+0.5*-9.8*t^2 equation:

Simplifying
-100 = 0t + 0.5 * -9.8t2

Anything times zero is zero.
-100 = 0t + 0.5 * -9.8t2

Multiply 0.5 * -9.8
-100 = 0 + -4.9t2
Remove the zero:
-100 = -4.9t2

Solving
-100 = -4.9t2

Solving for variable 't'.

Move all terms containing t to the left, all other terms to the right.

Add '4.9t2' to each side of the equation.
-100 + 4.9t2 = -4.9t2 + 4.9t2

Combine like terms: -4.9t2 + 4.9t2 = 0.0
-100 + 4.9t2 = 0.0

Add '100' to each side of the equation.
-100 + 100 + 4.9t2 = 0.0 + 100

Combine like terms: -100 + 100 = 0
0 + 4.9t2 = 0.0 + 100
4.9t2 = 0.0 + 100

Combine like terms: 0.0 + 100 = 100
4.9t2 = 100

Divide each side by '4.9'.
t2 = 20.40816327

Simplifying
t2 = 20.40816327

Take the square root of each side:
t = {-4.517539515, 4.517539515}