I think the expression 5x+6y+5 should read "5x+6y=5", and on that assumption we rewrite the first eqn: y=2x+15 and substitute into the second: 5x+6(2x+15)=5; 5x+12x+90=5, so 17x=-85 and x=-5. Now we calculate y=-2*5+15=5.
Check out the result: -10-5=-15; -25+30=5. Seems OK.