21=p^2-4p

Solution for 21=p^2-4p equation:

Simplifying
21 = p2 + -4p

Reorder the terms:
21 = -4p + p2

Solving
21 = -4p + p2

Solving for variable 'p'.

Reorder the terms:
21 + 4p + -1p2 = -4p + 4p + p2 + -1p2

Combine like terms: -4p + 4p = 0
21 + 4p + -1p2 = 0 + p2 + -1p2
21 + 4p + -1p2 = p2 + -1p2

Combine like terms: p2 + -1p2 = 0
21 + 4p + -1p2 = 0

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

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

Simplifying
7 + -1p = 0

Solving
7 + -1p = 0

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

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

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

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

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

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

Simplifying
3 + p = 0

Solving
3 + p = 0

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

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

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

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

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