21=t(t-4)

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

Simplifying
21 = t(t + -4)

Reorder the terms:
21 = t(-4 + t)
21 = (-4 * t + t * t)
21 = (-4t + t2)

Solving
21 = -4t + t2

Solving for variable 't'.

Reorder the terms:
21 + 4t + -1t2 = -4t + 4t + t2 + -1t2

Combine like terms: -4t + 4t = 0
21 + 4t + -1t2 = 0 + t2 + -1t2
21 + 4t + -1t2 = t2 + -1t2

Combine like terms: t2 + -1t2 = 0
21 + 4t + -1t2 = 0

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

Subproblem 1
Set the factor '(7 + -1t)' equal to zero and attempt to solve:

Simplifying
7 + -1t = 0

Solving
7 + -1t = 0

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

Add '-7' to each side of the equation.
7 + -7 + -1t = 0 + -7

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

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

Divide each side by '-1'.
t = 7

Simplifying
t = 7
Subproblem 2
Set the factor '(3 + t)' equal to zero and attempt to solve:

Simplifying
3 + t = 0

Solving
3 + t = 0

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

Add '-3' to each side of the equation.
3 + -3 + t = 0 + -3

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

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

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