The question is what both experssions best represent in the context of the problem.
The expresssion x best represents the width of the rectangle.
The expression x + 12 best reprents the lenght of the rectangle.
This is because if you call x the width, then the length is 12 more than that, i.e. 12 + x.
If you wanted to solve the problem now you could write
2*(width + length) = perimeter
2(x + 12 + x) = 24 and solve for x.
But the answer to the question is:
x best represents the width of the rectangle.
x + 12 best reprents the lenght of the rectangle.
