need help with implicit differentiation

Can someone please help me with this problem step by step? I have no idea where to start! thank you

The acceleration due to gravity, g at a distance r from the center of the earth is given by g= GM/r² where M is the mass of the earth and G is constant.
a. Find dg/dr

b. What is the practical interpretation of dg/dr?

c. You are told that M= 6*10^24 kilograms and G = 6.67*10^-20 while r is in kilometers. What is the value od dg/dr at the surface of the earth? (r=6400km)

d. What does this tell you about whether or not it is reasonable to assume g is constant near the surface of the earth?