I quote the problem as stated:

"Sam has to replace the molding around one of the doors in his house. He will need to replace the two lengths on each side of the door as well as the width above the door. The length is double the width, and he has 20 feet of molding to use on the door. Let x represent the width."

A. Write an expression that represents the length in terms of the width X.

eq. 1)  x = L1 + width + L2

eq. 2)  L1 = 2 * width

eq. 3)  L2 = 2 * width

substitute into eq. 1

x = (2 * width) + width + (2 * width)

x = 5 * width

B. Write an equation to find the length of the door.

Using the equation for x, above: divide both sides by of the equation by 5:

x / 5 = (5 * width) / 5

x / 5 = width

or

width = x / 5

C. Solve for the width of the door.

Assuming that we are required to use the entire 20 feet of molding, which is not stated as a requirement, but the problem cannot be solved unless that is true.

width = 20 feet / 5 = 4 feet

That is a very wide door...