3x+9y=15 and 2x-9y=-20
Notice that the coefficient of y-variable in each equation is the opposite of the other (9 and -9) . If you add them, they will be zero, thus eliminating them and leaving an equation with only x to solve for,
Step 1 - add the two equations
3x + 9y = 15
+ 2x - 9y = -20
5x = -5 Now divide each side by 5 to solve for x.
x = -1 Now substitute -1 for x in one of the original equations
to solve for y.
3(-1) + 9y = 15
-3 + 9y = 15 Add 3 to each side and then divide by 9
9y = 18
y = 2
The solution is (-1, 2)