Answer:
6x+y=30
-9x+3y=-18
Solving by substitution means that you have to express one of the variables (from one the equations) through the other one and substitute in the second equation.
In this example it is easier to express y through x in the first equation: y=30-6x. Put the expression for y in the second equation and solve for x.
-9x+3(30-6x)=-18 (divide by -3)
3x-30+6x=6
9x=36
x=4,
y=30-6x=30-24=6
Explanation: