21=s(s-4)

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

Simplifying
21 = s(s + -4)

Reorder the terms:
21 = s(-4 + s)
21 = (-4 * s + s * s)
21 = (-4s + s2)

Solving
21 = -4s + s2

Solving for variable 's'.

Reorder the terms:
21 + 4s + -1s2 = -4s + 4s + s2 + -1s2

Combine like terms: -4s + 4s = 0
21 + 4s + -1s2 = 0 + s2 + -1s2
21 + 4s + -1s2 = s2 + -1s2

Combine like terms: s2 + -1s2 = 0
21 + 4s + -1s2 = 0

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

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

Simplifying
7 + -1s = 0

Solving
7 + -1s = 0

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

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

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

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

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

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

Simplifying
3 + s = 0

Solving
3 + s = 0

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

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

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

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

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