z^2+8z-21=0

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Solution for z^2+8z-21=0 equation: 


Simplifying
z2 + 8z + -21 = 0

Reorder the terms:
-21 + 8z + z2 = 0

Solving
-21 + 8z + z2 = 0

Solving for variable 'z'.

Begin completing the square.

Move the constant term to the right:

Add '21' to each side of the equation.
-21 + 8z + 21 + z2 = 0 + 21

Reorder the terms:
-21 + 21 + 8z + z2 = 0 + 21

Combine like terms: -21 + 21 = 0
0 + 8z + z2 = 0 + 21
8z + z2 = 0 + 21

Combine like terms: 0 + 21 = 21
8z + z2 = 21

The z term is 8z.  Take half its coefficient (4).
Square it (16) and add it to both sides.

Add '16' to each side of the equation.
8z + 16 + z2 = 21 + 16

Reorder the terms:
16 + 8z + z2 = 21 + 16

Combine like terms: 21 + 16 = 37
16 + 8z + z2 = 37

Factor a perfect square on the left side:
(z + 4)(z + 4) = 37

Calculate the square root of the right side: 6.08276253

Break this problem into two subproblems by setting 
(z + 4) equal to 6.08276253 and -6.08276253.


Subproblem 1
z + 4 = 6.08276253

Simplifying
z + 4 = 6.08276253

Reorder the terms:
4 + z = 6.08276253

Solving
4 + z = 6.08276253

Solving for variable 'z'.

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

Add '-4' to each side of the equation.
4 + -4 + z = 6.08276253 + -4

Combine like terms: 4 + -4 = 0
0 + z = 6.08276253 + -4
z = 6.08276253 + -4

Combine like terms: 6.08276253 + -4 = 2.08276253
z = 2.08276253

Simplifying
z = 2.08276253


Subproblem 2
z + 4 = -6.08276253

Simplifying
z + 4 = -6.08276253

Reorder the terms:
4 + z = -6.08276253

Solving
4 + z = -6.08276253

Solving for variable 'z'.

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

Add '-4' to each side of the equation.
4 + -4 + z = -6.08276253 + -4

Combine like terms: 4 + -4 = 0
0 + z = -6.08276253 + -4
z = -6.08276253 + -4

Combine like terms: -6.08276253 + -4 = -10.08276253
z = -10.08276253

Simplifying
z = -10.08276253


Solution
The solution to the problem is based on the solutions
from the subproblems.
z = {2.08276253, -10.08276253}

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