Putting the equations into slope-intercept form,
\(y = -\frac{Ax}{B} + \frac{3}{B}\)
\(y = -\frac{x}{3} - \frac{5}{3}\)
Both lines are parallel, so \(-\frac{x}{3} = -\frac{Ax}{B}\). Simplify to find that \(\frac{A}{B} = \frac{1}{3}\), meaning that the slope is \(-\frac{1}{3}\).
\(y = -\frac{x}{3} + \frac{3}{B}\)
Plug in (-7, 2).
\(2 = -\frac{-7}{3} + \frac{3}{B}\)
\(6B = 7B + 9\)
\(B = -9\)
Because A and B are in ratio \(\frac{A}{B} = \frac{1}{3}\), B = -9 and A = -3. So B - A = -9 - (-3) = -6