90=0.81x^2

Solution for 90=0.81x^2 equation:

Simplifying
90 = 0.81x2

Solving
90 = 0.81x2

Solving for variable 'x'.

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

Add '-0.81x2' to each side of the equation.
90 + -0.81x2 = 0.81x2 + -0.81x2

Combine like terms: 0.81x2 + -0.81x2 = 0.00
90 + -0.81x2 = 0.00

Add '-90' to each side of the equation.
90 + -90 + -0.81x2 = 0.00 + -90

Combine like terms: 90 + -90 = 0
0 + -0.81x2 = 0.00 + -90
-0.81x2 = 0.00 + -90

Combine like terms: 0.00 + -90 = -90
-0.81x2 = -90

Divide each side by '-0.81'.
x2 = 111.1111111

Simplifying
x2 = 111.1111111

Take the square root of each side:
x = {-10.540925533, 10.540925533}