Your second equation is x - 2y = 9
If we set a = x - 3 then by adding 3 to each side we have x = a + 3
If we set b = y + 3 then by subtracting 3 from each side we have y = b - 3
Now we replace x and y in x - 2y = 9,
so (a + 3) - 2(b - 3) = 9
or a + 3 -2b + 6 = 9
or a - 2b + 9 = 9
or a - 2b = 0
.