Hey everyone, I need a little bit of help with this question so any help would be much appreciated.

Equation: $\displaystyle u_y -ku_{xx}=f(x,y)$ for $\displaystyle 0 \le x \le A$
$\displaystyle u(x,0)=g(x)$, $\displaystyle u(0,y)=h(y)$, $\displaystyle u(A,y)= j(y)$

Show the weak max principle applies to $\displaystyle u$ if $\displaystyle f \le 0$.

Any hints on how to do this question?