add these equations to eliminate y:
3x = -6
solve for x:
3x = -6. divide both sides by 3. x = -2
substitute x in the equation to find y:
9(-2) + 6y = 18.
6y - 18 = 18. add 18 to both sides.
6y = 36. divide both sides by 6. y = 6
end result:
x = -2
y = 6