0=r^2-21.16

Solution for 0=r^2-21.16 equation:

Simplifying
0 = r2 + -21.16

Reorder the terms:
0 = -21.16 + r2

Solving
0 = -21.16 + r2

Solving for variable 'r'.

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

Add '-1r2' to each side of the equation.
0 + -1r2 = -21.16 + r2 + -1r2
Remove the zero:
-1r2 = -21.16 + r2 + -1r2

Combine like terms: r2 + -1r2 = 0
-1r2 = -21.16 + 0
-1r2 = -21.16

Divide each side by '-1'.
r2 = 21.16

Simplifying
r2 = 21.16

Take the square root of each side:
r = {-4.6, 4.6}