Kinda confused about this question:

Zeta plans to fence off a rectangular field along the bank of a river; no fence is needed along the river. Fencing cost $2 per foot for the ends and $3 per foot for the side parallel to the river. Zeta's budge is $900

1. Let x be the length of the ide of the field that lies along the river. Explain why the formula A(x) = x(225 - .75x) describes the area of the field.

I know that the area of a rectangle would be length * width... which would explain where x is coming from but im not sure where the stuff inside the parenthesis are coming from. I think .75 would mean that the width would be 75% of the length but not sure how to get 225.