x^2-21=3(5-x^2)

Solution for x^2-21=3(5-x^2) equation:

Simplifying
x2 + -21 = 3(5 + -1x2)

Reorder the terms:
-21 + x2 = 3(5 + -1x2)
-21 + x2 = (5 * 3 + -1x2 * 3)
-21 + x2 = (15 + -3x2)

Solving
-21 + x2 = 15 + -3x2

Solving for variable 'x'.

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

Add '3x2' to each side of the equation.
-21 + x2 + 3x2 = 15 + -3x2 + 3x2

Combine like terms: x2 + 3x2 = 4x2
-21 + 4x2 = 15 + -3x2 + 3x2

Combine like terms: -3x2 + 3x2 = 0
-21 + 4x2 = 15 + 0
-21 + 4x2 = 15

Add '21' to each side of the equation.
-21 + 21 + 4x2 = 15 + 21

Combine like terms: -21 + 21 = 0
0 + 4x2 = 15 + 21
4x2 = 15 + 21

Combine like terms: 15 + 21 = 36
4x2 = 36

Divide each side by '4'.
x2 = 9

Simplifying
x2 = 9

Take the square root of each side:
x = {-3, 3}