You are given two lines, the blue one and the one with the letters and are told to find the length of <em>UV</em><em> </em>(which would just be from point U to point V)
The blue line seems to represent the total length of the black line and the length of <em>VW</em><em> </em>is already shown. Since the total length is 16 and we alredy know that VW is 4, we'll subtract the two to find the length of UV