21=b(b-4)

Solution for 21=b(b-4) equation:

Simplifying
21 = b(b + -4)

Reorder the terms:
21 = b(-4 + b)
21 = (-4 * b + b * b)
21 = (-4b + b2)

Solving
21 = -4b + b2

Solving for variable 'b'.

Reorder the terms:
21 + 4b + -1b2 = -4b + 4b + b2 + -1b2

Combine like terms: -4b + 4b = 0
21 + 4b + -1b2 = 0 + b2 + -1b2
21 + 4b + -1b2 = b2 + -1b2

Combine like terms: b2 + -1b2 = 0
21 + 4b + -1b2 = 0

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

Subproblem 1
Set the factor '(7 + -1b)' equal to zero and attempt to solve:

Simplifying
7 + -1b = 0

Solving
7 + -1b = 0

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

Add '-7' to each side of the equation.
7 + -7 + -1b = 0 + -7

Combine like terms: 7 + -7 = 0
0 + -1b = 0 + -7
-1b = 0 + -7

Combine like terms: 0 + -7 = -7
-1b = -7

Divide each side by '-1'.
b = 7

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

Simplifying
3 + b = 0

Solving
3 + b = 0

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

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

Combine like terms: 3 + -3 = 0
0 + b = 0 + -3
b = 0 + -3

Combine like terms: 0 + -3 = -3
b = -3

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