We know a rectangle has two congruent pairs of sides.

So, we are trying to find what multiplies to 20 and adds to 9 (half of 18 because we are only counting one pair).

x*y = 20

x + y = 9

The factors of 20 are 1,2,4,5,10,20 and two of those add up to 9.

When we look carefully we find those two are 4 and 5.

Sooo the dimensions of the rectangle are 4 and 5.

(Tell me if anything is wrong here)