15=0.5*9.8*t^2

Solution for 15=0.5*9.8*t^2 equation:

Simplifying
15 = 0.5 * 9.8t2

Multiply 0.5 * 9.8
15 = 4.9t2

Solving
15 = 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.
15 + -4.9t2 = 4.9t2 + -4.9t2

Combine like terms: 4.9t2 + -4.9t2 = 0.0
15 + -4.9t2 = 0.0

Add '-15' to each side of the equation.
15 + -15 + -4.9t2 = 0.0 + -15

Combine like terms: 15 + -15 = 0
0 + -4.9t2 = 0.0 + -15
-4.9t2 = 0.0 + -15

Combine like terms: 0.0 + -15 = -15
-4.9t2 = -15

Divide each side by '-4.9'.
t2 = 3.06122449

Simplifying
t2 = 3.06122449

Take the square root of each side:
t = {-1.749635531, 1.749635531}