One afternoon, a couple walks three-fourths of the way around a circular lake, the radius of which is 2.45 km. They start at the west side of the lake and head due south to begin with. (a) What is the distance they travel? (b) What is the magnitude of the couple’s displacement? (c) What is the direction (relative to due east) of the couple’s displacement?

I suggest you draw a sketch of the situation - this will help you see what's going on. And remember that the length of the circumference of a circle is C = 2 \pi R.