x = -3
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The following steps are taken to isolate x on one side of the equation in order to set it equal to a value on the other side of the equation.
4x + 3 = -9
Cancel out the 3 by subtracting it from both sides of the equation (since it is positive).
4x + 3 - 3 = -9 - 3
4x = -12
Cancel out the 4 (from 4x) by dividing both sides of the equation by it (since it is currently being multiplied).
4x / 4 = -12 / 4
x = -3
Variable x is now isolated on the left side of the equation, leaving the right side equal to x.