x^2+3x=81

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Solution for x^2+3x=81 equation:


Simplifying
x2 + 3x = 81

Reorder the terms:
3x + x2 = 81

Solving
3x + x2 = 81

Solving for variable 'x'.

Reorder the terms:
-81 + 3x + x2 = 81 + -81

Combine like terms: 81 + -81 = 0
-81 + 3x + x2 = 0

Begin completing the square.

Move the constant term to the right:

Add '81' to each side of the equation.
-81 + 3x + 81 + x2 = 0 + 81

Reorder the terms:
-81 + 81 + 3x + x2 = 0 + 81

Combine like terms: -81 + 81 = 0
0 + 3x + x2 = 0 + 81
3x + x2 = 0 + 81

Combine like terms: 0 + 81 = 81
3x + x2 = 81

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 = 81 + 2.25

Reorder the terms:
2.25 + 3x + x2 = 81 + 2.25

Combine like terms: 81 + 2.25 = 83.25
2.25 + 3x + x2 = 83.25

Factor a perfect square on the left side:
(x + 1.5)(x + 1.5) = 83.25

Calculate the square root of the right side: 9.124143795

Break this problem into two subproblems by setting 
(x + 1.5) equal to 9.124143795 and -9.124143795.

Subproblem 1

x + 1.5 = 9.124143795 Simplifying x + 1.5 = 9.124143795 Reorder the terms: 1.5 + x = 9.124143795 Solving 1.5 + x = 9.124143795 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 = 9.124143795 + -1.5 Combine like terms: 1.5 + -1.5 = 0.0 0.0 + x = 9.124143795 + -1.5 x = 9.124143795 + -1.5 Combine like terms: 9.124143795 + -1.5 = 7.624143795 x = 7.624143795 Simplifying x = 7.624143795

Subproblem 2

x + 1.5 = -9.124143795 Simplifying x + 1.5 = -9.124143795 Reorder the terms: 1.5 + x = -9.124143795 Solving 1.5 + x = -9.124143795 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 = -9.124143795 + -1.5 Combine like terms: 1.5 + -1.5 = 0.0 0.0 + x = -9.124143795 + -1.5 x = -9.124143795 + -1.5 Combine like terms: -9.124143795 + -1.5 = -10.624143795 x = -10.624143795 Simplifying x = -10.624143795

Solution

The solution to the problem is based on the solutions from the subproblems. x = {7.624143795, -10.624143795}

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