This question is basically: how many different pairs of numbers multiply up to 18?
(This is a simpler form of the question because the question is just asking how many shapes of a rectangle can Jenn create with 18 blocks)
So, the numbers that multiply up to 18 are:
1 x 18
2 x 9
3 x 6
So, the answer is: Jenn can make the model of the park in 3 different ways.