You are trying to solve the length of the house. Let x equal the length of the house. The length of the house is 2 1/2, or 2.5 times its width. The width of the house is 30 feet.
Your equation should look like this:
x = 30(2.5)
Mutiply 30 ft. times 2.5 to get 750 ft.
x = 750 ft.
Don't forget to use your unit of measurement in the answer! You are working in feet.