Use linear combination to solve this. It is much faster than substitution. I would start by multiplying the bottom equation by -1.
4x-7y=3
-x+7y=15
Then, add the two equations together. The result is:
3x=18.
You've eliminated one variable. Now you can solve for the other one.
Divide both sides by 3 to get
x=6
Plug this into one of the original equations, then solve for y
-(6)+7y=15
7y=21
y=3
Check your work by plugging these values into both equations to see if they remain true