3x^2+21x-8.1=0

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Solution for 3x^2+21x-8.1=0 equation: 


Simplifying
3x2 + 21x + -8.1 = 0

Reorder the terms:
-8.1 + 21x + 3x2 = 0

Solving
-8.1 + 21x + 3x2 = 0

Solving for variable 'x'.

Begin completing the square.  Divide all terms by
3 the coefficient of the squared term: 

Divide each side by '3'.
-2.7 + 7x + x2 = 0

Move the constant term to the right:

Add '2.7' to each side of the equation.
-2.7 + 7x + 2.7 + x2 = 0 + 2.7

Reorder the terms:
-2.7 + 2.7 + 7x + x2 = 0 + 2.7

Combine like terms: -2.7 + 2.7 = 0.0
0.0 + 7x + x2 = 0 + 2.7
7x + x2 = 0 + 2.7

Combine like terms: 0 + 2.7 = 2.7
7x + x2 = 2.7

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 = 2.7 + 12.25

Reorder the terms:
12.25 + 7x + x2 = 2.7 + 12.25

Combine like terms: 2.7 + 12.25 = 14.95
12.25 + 7x + x2 = 14.95

Factor a perfect square on the left side:
(x + 3.5)(x + 3.5) = 14.95

Calculate the square root of the right side: 3.866522986

Break this problem into two subproblems by setting 
(x + 3.5) equal to 3.866522986 and -3.866522986.


Subproblem 1
x + 3.5 = 3.866522986

Simplifying
x + 3.5 = 3.866522986

Reorder the terms:
3.5 + x = 3.866522986

Solving
3.5 + x = 3.866522986

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 = 3.866522986 + -3.5

Combine like terms: 3.5 + -3.5 = 0.0
0.0 + x = 3.866522986 + -3.5
x = 3.866522986 + -3.5

Combine like terms: 3.866522986 + -3.5 = 0.366522986
x = 0.366522986

Simplifying
x = 0.366522986


Subproblem 2
x + 3.5 = -3.866522986

Simplifying
x + 3.5 = -3.866522986

Reorder the terms:
3.5 + x = -3.866522986

Solving
3.5 + x = -3.866522986

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 = -3.866522986 + -3.5

Combine like terms: 3.5 + -3.5 = 0.0
0.0 + x = -3.866522986 + -3.5
x = -3.866522986 + -3.5

Combine like terms: -3.866522986 + -3.5 = -7.366522986
x = -7.366522986

Simplifying
x = -7.366522986


Solution
The solution to the problem is based on the solutions
from the subproblems.
x = {0.366522986, -7.366522986}

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