I assume they mean that only whole numbers are allowed (they should have said so!).
You can solve these by writing a factor table, like this:
1 2 3 4 6
36 18 12 9 6
These are all the factors of 36. This means, "Multiply the top number by the bottom number, and you'll get 36." Since you find the area of a rectangle by multiplying the length by the width, then these factor pairs are also your rectangles.