h^2-4h-21=0

Solution for h^2-4h-21=0 equation:

Simplifying
h2 + -4h + -21 = 0

Reorder the terms:
-21 + -4h + h2 = 0

Solving
-21 + -4h + h2 = 0

Solving for variable 'h'.

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

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

Simplifying
-3 + -1h = 0

Solving
-3 + -1h = 0

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

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

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

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

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

Simplifying
h = -3
Subproblem 2
Set the factor '(7 + -1h)' equal to zero and attempt to solve:

Simplifying
7 + -1h = 0

Solving
7 + -1h = 0

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

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

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

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

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

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