400=.5x(2x)

Solution for 400=.5x(2x) equation:

Simplifying
400 = 0.5x(2x)

Remove parenthesis around (2x)
400 = 0.5x * 2x

Reorder the terms for easier multiplication:
400 = 0.5 * 2x * x

Multiply 0.5 * 2
400 = 1x * x

Multiply x * x
400 = 1x2

Solving
400 = 1x2

Solving for variable 'x'.

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

Add '-1x2' to each side of the equation.
400 + -1x2 = 1x2 + -1x2

Combine like terms: 1x2 + -1x2 = 0
400 + -1x2 = 0

Add '-400' to each side of the equation.
400 + -400 + -1x2 = 0 + -400

Combine like terms: 400 + -400 = 0
0 + -1x2 = 0 + -400
-1x2 = 0 + -400

Combine like terms: 0 + -400 = -400
-1x2 = -400

Divide each side by '-1'.
x2 = 400

Simplifying
x2 = 400

Take the square root of each side:
x = {-20, 20}