r^2-2-21=0

Solution for r^2-2-21=0 equation:

Simplifying
r2 + -2 + -21 = 0

Reorder the terms:
-2 + -21 + r2 = 0

Combine like terms: -2 + -21 = -23
-23 + r2 = 0

Solving
-23 + r2 = 0

Solving for variable 'r'.

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

Add '23' to each side of the equation.
-23 + 23 + r2 = 0 + 23

Combine like terms: -23 + 23 = 0
0 + r2 = 0 + 23
r2 = 0 + 23

Combine like terms: 0 + 23 = 23
r2 = 23

Simplifying
r2 = 23

Take the square root of each side:
r = {-4.795831523, 4.795831523}