21=v^2+4v

Solution for 21=v^2+4v equation:

Simplifying
21 = v2 + 4v

Reorder the terms:
21 = 4v + v2

Solving
21 = 4v + v2

Solving for variable 'v'.

Reorder the terms:
21 + -4v + -1v2 = 4v + -4v + v2 + -1v2

Combine like terms: 4v + -4v = 0
21 + -4v + -1v2 = 0 + v2 + -1v2
21 + -4v + -1v2 = v2 + -1v2

Combine like terms: v2 + -1v2 = 0
21 + -4v + -1v2 = 0

Factor a trinomial.
(3 + -1v)(7 + v) = 0

Subproblem 1
Set the factor '(3 + -1v)' equal to zero and attempt to solve:

Simplifying
3 + -1v = 0

Solving
3 + -1v = 0

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

Add '-3' to each side of the equation.
3 + -3 + -1v = 0 + -3

Combine like terms: 3 + -3 = 0
0 + -1v = 0 + -3
-1v = 0 + -3

Combine like terms: 0 + -3 = -3
-1v = -3

Divide each side by '-1'.
v = 3

Simplifying
v = 3
Subproblem 2
Set the factor '(7 + v)' equal to zero and attempt to solve:

Simplifying
7 + v = 0

Solving
7 + v = 0

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

Add '-7' to each side of the equation.
7 + -7 + v = 0 + -7

Combine like terms: 7 + -7 = 0
0 + v = 0 + -7
v = 0 + -7

Combine like terms: 0 + -7 = -7
v = -7

Simplifying
v = -7
Solution
v = {3, -7}