For the equation
.. x - y = 4
you can divide by 4 to put the equation into intercept form. This form shows you both interecepts at once.
.. x/4 +y/(-4) = 1
The x-intercept is (4, 0).
The y-intercept is (0, -4).
The graph is shown below.
If I understand your question correctly then it's simple all you have to do is use the numbers in the place of the b and c so the question would be what is 8-7 which equals 1