3x^2-21x-12=0

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Solution for 3x^2-21x-12=0 equation: 


Simplifying
3x2 + -21x + -12 = 0

Reorder the terms:
-12 + -21x + 3x2 = 0

Solving
-12 + -21x + 3x2 = 0

Solving for variable 'x'.

Factor out the Greatest Common Factor (GCF), '3'.
3(-4 + -7x + x2) = 0

Ignore the factor 3.

Subproblem 1
Set the factor '(-4 + -7x + x2)' equal to zero and attempt to solve:

Simplifying
-4 + -7x + x2 = 0

Solving
-4 + -7x + x2 = 0

Begin completing the square.

Move the constant term to the right:

Add '4' to each side of the equation.
-4 + -7x + 4 + x2 = 0 + 4

Reorder the terms:
-4 + 4 + -7x + x2 = 0 + 4

Combine like terms: -4 + 4 = 0
0 + -7x + x2 = 0 + 4
-7x + x2 = 0 + 4

Combine like terms: 0 + 4 = 4
-7x + x2 = 4

The x term is -7x.  Take half its coefficient (-3.5).
Square it (12.25) and add it to both sides.

Add '12.25' to each side of the equation.
-7x + 12.25 + x2 = 4 + 12.25

Reorder the terms:
12.25 + -7x + x2 = 4 + 12.25

Combine like terms: 4 + 12.25 = 16.25
12.25 + -7x + x2 = 16.25

Factor a perfect square on the left side:
(x + -3.5)(x + -3.5) = 16.25

Calculate the square root of the right side: 4.031128874

Break this problem into two subproblems by setting 
(x + -3.5) equal to 4.031128874 and -4.031128874.


Subproblem 1
x + -3.5 = 4.031128874

Simplifying
x + -3.5 = 4.031128874

Reorder the terms:
-3.5 + x = 4.031128874

Solving
-3.5 + x = 4.031128874

Solving for variable 'x'.

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

Add '3.5' to each side of the equation.
-3.5 + 3.5 + x = 4.031128874 + 3.5

Combine like terms: -3.5 + 3.5 = 0.0
0.0 + x = 4.031128874 + 3.5
x = 4.031128874 + 3.5

Combine like terms: 4.031128874 + 3.5 = 7.531128874
x = 7.531128874

Simplifying
x = 7.531128874


Subproblem 2
x + -3.5 = -4.031128874

Simplifying
x + -3.5 = -4.031128874

Reorder the terms:
-3.5 + x = -4.031128874

Solving
-3.5 + x = -4.031128874

Solving for variable 'x'.

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

Add '3.5' to each side of the equation.
-3.5 + 3.5 + x = -4.031128874 + 3.5

Combine like terms: -3.5 + 3.5 = 0.0
0.0 + x = -4.031128874 + 3.5
x = -4.031128874 + 3.5

Combine like terms: -4.031128874 + 3.5 = -0.531128874
x = -0.531128874

Simplifying
x = -0.531128874


Solution
The solution to the problem is based on the solutions
from the subproblems.
x = {7.531128874, -0.531128874}
Solution
x = {7.531128874, -0.531128874}

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