Lets make the graph of our systems of equations:
The blue line correspond to the equation x-y=3 and the red one correspond to 7x-y=-3. The lines intersect at point (-1,-4), then, the solution of the system is (-1,-4).
I’d be more than happy to help you if you could give me the graph
( [72-60] / 72 )*100 = 50 / 3 = 16.7 %