Help with GPS problem

Can I get some help with this problem.

1. Set up your own 2D planar trilateration problem. Place the SVs at (0,300) (100,400), (700,400), and (800,300). Generate a range measurement for a base station at (400,0) and a user at (401,0)

a) Solve for the position of the user using 2 SVs and then 4 SVs assuming no clock errors

b) Solve for the position of the user assuming you need to solve for the user clock bias

c) Calculate a differential solution between the base and user using a single difference model and assuming you must solve for a clock bias between the base station and user