navigation word problem: help

Earth has been charted with vertical and horizontal lines so that points can be named with coordinates. The horizontal lines are called latitude lines. The equator is latitude line 0 degrees. Parallel lines are numbered up to 90 degrees to the north and to the south. The length of any parallel of latutude is equal to the distance around Earth times the cosine of the latutude angle, if we assume a spherical Earth.

a. If the radius of Earth is about 6400 km, which latitude lines are 5543 km long?

b. What is the length of the 90 degree parallel? Why?