How to convert two conflicting objective functions into a single objective function

Question

I have two objective functions say f1 where I have to minimize (X+Y) and another function f2 where I have to maximize (A-B). The two functions are conflicting.

I need to convert them into a minimization problem involving one single objective function using the weighed sum approach where the final form would become f3 = Min {uf1 (+ or-) vf2} where u+v=1.

I am not sure what would be the sign before vf2 ? I wonder if it should be f3 = Min {uf1 + vf2} or f3 = Min {uf1 - v*f2}.

Thanks in advance.