x=7y + 6
-3x + 21y=-18
-3(7y+6) + 21y = -18
-21y -18 + 21y = -18
-21y + 21y= 0
-18 = -18
any solution or value of y will make this true
for the next one,
y=3x+2
6x-2y=10
6x-2(3x+2)=10
6x- 6x -4 = 10
-4=10, but -4 does not equal 10, which means that this one has no solution
