The equations are

                           Equation (1)

                                Equation (2)

                               Equation (3)

Substitute equation (3) in equation (1).

Apply subtraction property of equality: if a = b, then .

Subtract 6 from each side.

Substitute in equation (2).

Apply subtraction property of equality: if a = b, then .

Subtract 5 from each side.

Substitute and in equation (1).

Apply subtraction property of equality: if a = b, then .

Subtract 12 from each side.

The solution is .