How do i express the area as a function of its length.

The rectangle has a width of 6 inches less than 4 times it length.

w=4L-6

A=l(4l-6)
A=4L^2 -6L
A= 2L(2L-3)

get rid of w

Come up with an expression for the area.

Let's call the length L, and the width w.

4L-6 = w.

Area = L*w = L*(4L-6) = 4L^2 - 6L.

Now you have an expression of the area only in terms of the length, L. Coming up with algebraic expressions based on prose is a difficult but powerful skill.