Answer:
Hi, Victoria. When you are dealing with a system of equations in three variables, you want to see first if there is anyway that you can eliminate two variables in one fell swoop. Remember that for any one given equation, you can multiply both sides by the same number without making the equation untrue. Also, remember that you can add or subtract two equations.
If we add the first and second equations, we will eliminate both the x and the y.
(2x + y + z) + (2x - y - z) = (3) + (9)