What separates the crust to the mantle?

What is relative velocity? in which condition is relative velocity known by adding the velocity of the first body and that of the second body

It is really hard for me right know

The period of a pendulum varies directly as the square root of the length of the pendulum and inversely as the square root of the acceleration due to gravity. find the period when the length is 144 cm and the acceleration due to gravity is 980 cm per second squared, if the period is 7pi seconds when the length is 289 cm and the acceleration due to gravity is 980 cm per second squared.