d^2=21-4d

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

Simplifying
d2 = 21 + -4d

Solving
d2 = 21 + -4d

Solving for variable 'd'.

Reorder the terms:
-21 + 4d + d2 = 21 + -4d + -21 + 4d

Reorder the terms:
-21 + 4d + d2 = 21 + -21 + -4d + 4d

Combine like terms: 21 + -21 = 0
-21 + 4d + d2 = 0 + -4d + 4d
-21 + 4d + d2 = -4d + 4d

Combine like terms: -4d + 4d = 0
-21 + 4d + d2 = 0

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

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

Simplifying
-7 + -1d = 0

Solving
-7 + -1d = 0

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

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

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

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

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

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

Simplifying
3 + -1d = 0

Solving
3 + -1d = 0

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

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

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

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

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

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