5x^2-15x-21=0

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Solution for 5x^2-15x-21=0 equation:


Simplifying
5x2 + -15x + -21 = 0

Reorder the terms:
-21 + -15x + 5x2 = 0

Solving
-21 + -15x + 5x2 = 0

Solving for variable 'x'.

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

Divide each side by '5'.
-4.2 + -3x + x2 = 0

Move the constant term to the right:

Add '4.2' to each side of the equation.
-4.2 + -3x + 4.2 + x2 = 0 + 4.2

Reorder the terms:
-4.2 + 4.2 + -3x + x2 = 0 + 4.2

Combine like terms: -4.2 + 4.2 = 0.0
0.0 + -3x + x2 = 0 + 4.2
-3x + x2 = 0 + 4.2

Combine like terms: 0 + 4.2 = 4.2
-3x + x2 = 4.2

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 = 4.2 + 2.25

Reorder the terms:
2.25 + -3x + x2 = 4.2 + 2.25

Combine like terms: 4.2 + 2.25 = 6.45
2.25 + -3x + x2 = 6.45

Factor a perfect square on the left side:
(x + -1.5)(x + -1.5) = 6.45

Calculate the square root of the right side: 2.53968502

Break this problem into two subproblems by setting 
(x + -1.5) equal to 2.53968502 and -2.53968502.

Subproblem 1

x + -1.5 = 2.53968502 Simplifying x + -1.5 = 2.53968502 Reorder the terms: -1.5 + x = 2.53968502 Solving -1.5 + x = 2.53968502 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 = 2.53968502 + 1.5 Combine like terms: -1.5 + 1.5 = 0.0 0.0 + x = 2.53968502 + 1.5 x = 2.53968502 + 1.5 Combine like terms: 2.53968502 + 1.5 = 4.03968502 x = 4.03968502 Simplifying x = 4.03968502

Subproblem 2

x + -1.5 = -2.53968502 Simplifying x + -1.5 = -2.53968502 Reorder the terms: -1.5 + x = -2.53968502 Solving -1.5 + x = -2.53968502 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 = -2.53968502 + 1.5 Combine like terms: -1.5 + 1.5 = 0.0 0.0 + x = -2.53968502 + 1.5 x = -2.53968502 + 1.5 Combine like terms: -2.53968502 + 1.5 = -1.03968502 x = -1.03968502 Simplifying x = -1.03968502

Solution

The solution to the problem is based on the solutions from the subproblems. x = {4.03968502, -1.03968502}

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