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Right answer is -14

ADDITION METHOD

The whole problem with solving a system of equations is that you cannot solve an equation that has two unknowns in it. You need an equation with only one variable so that you can isolate the variable on one side of the equation. Both methods that we will look at are techniques for eliminating one of the variables to give you an equation in just one unknown, which you can then solve by the usual methods.
The first method of solving systems of linear equations is the addition method, in which the two equations are added together to eliminate one of the variables.
Adding the equations means that we add the left sides of the two equations together, and we add the right sides together. This is legal because of the Addition Principle, which says that we can add the same amount to both sides of an equation. Since the left and right sides of any equation are equal to each other, we are indeed adding the same amount to both sides of an equation.