90=(5x)(4x)

Solution for 90=(5x)(4x) equation:

Simplifying
90 = (5x)(4x)

Remove parenthesis around (5x)
90 = 5x(4x)

Remove parenthesis around (4x)
90 = 5x * 4x

Reorder the terms for easier multiplication:
90 = 5 * 4x * x

Multiply 5 * 4
90 = 20x * x

Multiply x * x
90 = 20x2

Solving
90 = 20x2

Solving for variable 'x'.

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

Add '-20x2' to each side of the equation.
90 + -20x2 = 20x2 + -20x2

Combine like terms: 20x2 + -20x2 = 0
90 + -20x2 = 0

Add '-90' to each side of the equation.
90 + -90 + -20x2 = 0 + -90

Combine like terms: 90 + -90 = 0
0 + -20x2 = 0 + -90
-20x2 = 0 + -90

Combine like terms: 0 + -90 = -90
-20x2 = -90

Divide each side by '-20'.
x2 = 4.5

Simplifying
x2 = 4.5

Take the square root of each side:
x = {-2.121320344, 2.121320344}