Exact Equations & Substitutions · Challenge problem

The equation $(3xy + y^2)\,dx + (x^2 + 3xy)\,dy = 0$ has an integrating factor that depends only on $s = x + y$. Use it to solve the equation with $y(1) = 1$ for $x, y > 0$. The solution curve crosses the line $x + y = 4$ at a point with $y > x$. Find the $y$-coordinate of that point.