(r*r)+4-21=0

Solution for (r*r)+4-21=0 equation:

Simplifying
(r * r) + 4 + -21 = 0

Multiply r * r
(r2) + 4 + -21 = 0
r2 + 4 + -21 = 0

Reorder the terms:
4 + -21 + r2 = 0

Combine like terms: 4 + -21 = -17
-17 + r2 = 0

Solving
-17 + r2 = 0

Solving for variable 'r'.

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

Add '17' to each side of the equation.
-17 + 17 + r2 = 0 + 17

Combine like terms: -17 + 17 = 0
0 + r2 = 0 + 17
r2 = 0 + 17

Combine like terms: 0 + 17 = 17
r2 = 17

Simplifying
r2 = 17

Take the square root of each side:
r = {-4.123105626, 4.123105626}