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

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

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

Any hints on how to do this question?