Answer:
( x - y ) ( a - b ) = -1
Explanation:
ax + by = c and bx + ay = 1 + c
ax + by = c and bx + ay - 1 = c
And now we set c equivalent
ax + by = bx + ay - 1
ax - bx + by - ay = -1
x (a - b) + -y (a - b) = -1
(x - y) ( a - b) = -1
So we can leave it like that, or we can solve for one of the variables.
If we solve for y:
(x - y) ( a - b) = -1
(x - y) = -1 / (a - b)
-y = (-1 / (a - b)) - x
y = (1 / (a-b)) + x
Cheers.