2x(x+3)=21

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Solution for 2x(x+3)=21 equation:


Simplifying
2x(x + 3) = 21

Reorder the terms:
2x(3 + x) = 21
(3 * 2x + x * 2x) = 21
(6x + 2x2) = 21

Solving
6x + 2x2 = 21

Solving for variable 'x'.

Reorder the terms:
-21 + 6x + 2x2 = 21 + -21

Combine like terms: 21 + -21 = 0
-21 + 6x + 2x2 = 0

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

Divide each side by '2'.
-10.5 + 3x + x2 = 0

Move the constant term to the right:

Add '10.5' to each side of the equation.
-10.5 + 3x + 10.5 + x2 = 0 + 10.5

Reorder the terms:
-10.5 + 10.5 + 3x + x2 = 0 + 10.5

Combine like terms: -10.5 + 10.5 = 0.0
0.0 + 3x + x2 = 0 + 10.5
3x + x2 = 0 + 10.5

Combine like terms: 0 + 10.5 = 10.5
3x + x2 = 10.5

The x term is 3x.  Take half its coefficient (1.5).
Square it (2.25) and add it to both sides.

Add '2.25' to each side of the equation.
3x + 2.25 + x2 = 10.5 + 2.25

Reorder the terms:
2.25 + 3x + x2 = 10.5 + 2.25

Combine like terms: 10.5 + 2.25 = 12.75
2.25 + 3x + x2 = 12.75

Factor a perfect square on the left side:
(x + 1.5)(x + 1.5) = 12.75

Calculate the square root of the right side: 3.570714214

Break this problem into two subproblems by setting 
(x + 1.5) equal to 3.570714214 and -3.570714214.

Subproblem 1

x + 1.5 = 3.570714214 Simplifying x + 1.5 = 3.570714214 Reorder the terms: 1.5 + x = 3.570714214 Solving 1.5 + x = 3.570714214 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-1.5' to each side of the equation. 1.5 + -1.5 + x = 3.570714214 + -1.5 Combine like terms: 1.5 + -1.5 = 0.0 0.0 + x = 3.570714214 + -1.5 x = 3.570714214 + -1.5 Combine like terms: 3.570714214 + -1.5 = 2.070714214 x = 2.070714214 Simplifying x = 2.070714214

Subproblem 2

x + 1.5 = -3.570714214 Simplifying x + 1.5 = -3.570714214 Reorder the terms: 1.5 + x = -3.570714214 Solving 1.5 + x = -3.570714214 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-1.5' to each side of the equation. 1.5 + -1.5 + x = -3.570714214 + -1.5 Combine like terms: 1.5 + -1.5 = 0.0 0.0 + x = -3.570714214 + -1.5 x = -3.570714214 + -1.5 Combine like terms: -3.570714214 + -1.5 = -5.070714214 x = -5.070714214 Simplifying x = -5.070714214

Solution

The solution to the problem is based on the solutions from the subproblems. x = {2.070714214, -5.070714214}

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