r^2+21r+20=0

Solution for r^2+21r+20=0 equation:

Simplifying
r2 + 21r + 20 = 0

Reorder the terms:
20 + 21r + r2 = 0

Solving
20 + 21r + r2 = 0

Solving for variable 'r'.

Factor a trinomial.
(20 + r)(1 + r) = 0

Subproblem 1
Set the factor '(20 + r)' equal to zero and attempt to solve:

Simplifying
20 + r = 0

Solving
20 + r = 0

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

Add '-20' to each side of the equation.
20 + -20 + r = 0 + -20

Combine like terms: 20 + -20 = 0
0 + r = 0 + -20
r = 0 + -20

Combine like terms: 0 + -20 = -20
r = -20

Simplifying
r = -20
Subproblem 2
Set the factor '(1 + r)' equal to zero and attempt to solve:

Simplifying
1 + r = 0

Solving
1 + r = 0

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

Add '-1' to each side of the equation.
1 + -1 + r = 0 + -1

Combine like terms: 1 + -1 = 0
0 + r = 0 + -1
r = 0 + -1

Combine like terms: 0 + -1 = -1
r = -1

Simplifying
r = -1
Solution
r = {-20, -1}