4r^2-81=

Solution for 4r^2-81= equation:

Simplifying
4r2 + -81 = 0

Reorder the terms:
-81 + 4r2 = 0

Solving
-81 + 4r2 = 0

Solving for variable 'r'.

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

Add '81' to each side of the equation.
-81 + 81 + 4r2 = 0 + 81

Combine like terms: -81 + 81 = 0
0 + 4r2 = 0 + 81
4r2 = 0 + 81

Combine like terms: 0 + 81 = 81
4r2 = 81

Divide each side by '4'.
r2 = 20.25

Simplifying
r2 = 20.25

Take the square root of each side:
r = {-4.5, 4.5}