In order to find the equation, you need to find two things. The y intercept, and the slope. Let's start with slope.

Slope is y/x, so 1-(-5)/8-(-3) = 6/11

This means -A/B = 6/11 (I think some part of the copy and paste didn't show up for the "is positive" part)

This means A and B equal -6,11 or 6,-11

Now for C, We can use one of the coordinates given. Let's use (8,1). -48 + 11 = C, C = -37. Note that if we used the other A and B, C would be 37.

Hope that helped :)