21=(x+2)(x-2)

Solution for 21=(x+2)(x-2) equation:

Simplifying
21 = (x + 2)(x + -2)

Reorder the terms:
21 = (2 + x)(x + -2)

Reorder the terms:
21 = (2 + x)(-2 + x)

Multiply (2 + x) * (-2 + x)
21 = (2(-2 + x) + x(-2 + x))
21 = ((-2 * 2 + x * 2) + x(-2 + x))
21 = ((-4 + 2x) + x(-2 + x))
21 = (-4 + 2x + (-2 * x + x * x))
21 = (-4 + 2x + (-2x + x2))

Combine like terms: 2x + -2x = 0
21 = (-4 + 0 + x2)
21 = (-4 + x2)

Solving
21 = -4 + x2

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.
21 + -1x2 = -4 + x2 + -1x2

Combine like terms: x2 + -1x2 = 0
21 + -1x2 = -4 + 0
21 + -1x2 = -4

Add '-21' to each side of the equation.
21 + -21 + -1x2 = -4 + -21

Combine like terms: 21 + -21 = 0
0 + -1x2 = -4 + -21
-1x2 = -4 + -21

Combine like terms: -4 + -21 = -25
-1x2 = -25

Divide each side by '-1'.
x2 = 25

Simplifying
x2 = 25

Take the square root of each side:
x = {-5, 5}