We know if we combine 7x + 3y -1 and x we get 180 degrees
7x+3y-1+x=180
8x-3y=181
We also know that 7x+3y-1=10x-7y+9
-3x=-10y+10
-3x+10y=10
If we combine the 2 equations:
8x-3y=181
-3x+10y=10
24x-9y=543
-24x+80y=80
71y=623
Now you can do it from here. :)
