Question

Find two numbers A and B (with A≤B) whose difference is 36 and whose product is minimized

Answer 1

@danny_soo are you still around?

Answer 2

P=AB
A-B=36 <solve this equation for A and replace A in P=AB with what you found for A (in terms of B)
Then if you know calculus, I would differentiate
If you don't know calculus find the vertex of the parabola.