If f(x1)>f(x2) for every x1>x2, then what is the behaviour of f(x)?

Question

I believe that the answer is that f(x) is increasing but I do not know how to show my work for that

Answer 1

what is the slope?
(f(x1)-f(x2)) / (x1-x2)
both are positive, so the slope is always positive.