How do I get the particular solution of the inhomogeneous equation $u_{x} + u_{y} = 3$?

Question

So far, I'm easily able to derive the solution of the homogeneous equation $u_{x}+u_{y}=0$ using separation of variables. That is, since $u$ is constant on the line $y = x + C$, it follows that $u(x,y)=f(y-x)$. But I'm not sure how to proceed from there.