You have 48 ft of fencing, and the length of the fence needs to be three times it's width.
Find the dimensions (length and width) of the fence that would fit your given situation

2L+2W=48 and L=3W.

Plugging in 3W for L and we get

2(3W)+2W=48 \Rightarrow 8W=48 \Rightarrow W=6

and so L=3W=3(6)=18 .