SOLUTION: How do i put x-y = -3 in a y = mx+b form?

 x-y = -3

Subtract x from both sides:

 x-y = -3
-x       -x
-----------
  -y = -3-x

Consider that as

 -1y = -3-1x

Divide every number by the coefficient of y which is -1

 y = -x

Simplify

 y = 3-(-1)x
 y = 3+x

Reverse the terms on the right

 y = x+3

That can also be written 

 y = 1x+3

So that is in y=mx+b form, called the slope-intercept form.

The slope = m = 1, and the y-intercept is (0,b) = (0,3)

Edwin