This is one of the example problems from my final exam. Could you please explain how to solve this problem and others like it$?$ specifically can you explain how i identify what equation iI should be using?

The existence of the black hole at the center of the Milky Way was deduced by observing the orbital motion of stars close to it$.$

$($A$)$ Calculate the mass of a black hole that is able to keep stars orbiting in perfect circles with a period of $96$ hours at a distance of million km$.)$

$($B$)$ What is the speed of such a star?