Martin wants to build an additional closet in a corner of his bedroom. Because the closet will be in a corner, only two new walls need to be built. The total length of the two new walls must be 12 m. Martin wants the length of the closet to be twice as long as the width.

Let the function f(l) be the sum of the length and the width. Find the equation for f(l).

I'm not really sure how you go about answering this...

For this problem, let l be the length of the longer part of the wall and w be the shorter part of the wall. They must add up to 12 m, so, your first equation:

l + w = 12

That would be your function of the sum of the length and width and l+w the function.

To find the value of l and w, you need to consider the second piece of information, which says that l is twice the length of w, so, 2w must add up to be of the same length as l. You have your second equation:

l = 2w

You have two equations, which you can now solve simultaneously.

Hope it helped! :)