Welcome to Math Help Forum!
There are then feet of fencing remaining, that must be divided equally to form the other two sides of the rectangle. So that's feet for each side.
So the whole enclosure measures feet by feet, or square feet in area. There's our function, then, for part (a):
This is a quadratic expression in . I assume you've been shown a method to find its maximum value. So can you complete part (b) now?