2x^2+12x=21

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Solution for 2x^2+12x=21 equation:


Simplifying
2x2 + 12x = 21

Reorder the terms:
12x + 2x2 = 21

Solving
12x + 2x2 = 21

Solving for variable 'x'.

Reorder the terms:
-21 + 12x + 2x2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 12x + 2x2 = 0

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

Divide each side by '2'.
-10.5 + 6x + x2 = 0

Move the constant term to the right:

Add '10.5' to each side of the equation.
-10.5 + 6x + 10.5 + x2 = 0 + 10.5

Reorder the terms:
-10.5 + 10.5 + 6x + x2 = 0 + 10.5

Combine like terms: -10.5 + 10.5 = 0.0
0.0 + 6x + x2 = 0 + 10.5
6x + x2 = 0 + 10.5

Combine like terms: 0 + 10.5 = 10.5
6x + x2 = 10.5

The x term is 6x.  Take half its coefficient (3).
Square it (9) and add it to both sides.

Add '9' to each side of the equation.
6x + 9 + x2 = 10.5 + 9

Reorder the terms:
9 + 6x + x2 = 10.5 + 9

Combine like terms: 10.5 + 9 = 19.5
9 + 6x + x2 = 19.5

Factor a perfect square on the left side:
(x + 3)(x + 3) = 19.5

Calculate the square root of the right side: 4.415880433

Break this problem into two subproblems by setting 
(x + 3) equal to 4.415880433 and -4.415880433.

Subproblem 1

x + 3 = 4.415880433 Simplifying x + 3 = 4.415880433 Reorder the terms: 3 + x = 4.415880433 Solving 3 + x = 4.415880433 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-3' to each side of the equation. 3 + -3 + x = 4.415880433 + -3 Combine like terms: 3 + -3 = 0 0 + x = 4.415880433 + -3 x = 4.415880433 + -3 Combine like terms: 4.415880433 + -3 = 1.415880433 x = 1.415880433 Simplifying x = 1.415880433

Subproblem 2

x + 3 = -4.415880433 Simplifying x + 3 = -4.415880433 Reorder the terms: 3 + x = -4.415880433 Solving 3 + x = -4.415880433 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-3' to each side of the equation. 3 + -3 + x = -4.415880433 + -3 Combine like terms: 3 + -3 = 0 0 + x = -4.415880433 + -3 x = -4.415880433 + -3 Combine like terms: -4.415880433 + -3 = -7.415880433 x = -7.415880433 Simplifying x = -7.415880433

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.415880433, -7.415880433}

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