How do you calculate the mass of a supermassive black hole?
You can calculate the mass of a supermassive black hole from the orbit of a star orbiting the black hole.
You can calculate the mass of an object from the orbit of a body orbiting around it. A form of Kepler's third law is
Evidence for the supermassive black hole at the centre of our galaxy, Sagittarius A*, came from the discovery of several fast moving stars orbiting the centre of the galaxy. One of the first ones to be discovered is called S2. It has a period of about 15.56 years and a semi-major axis of about 1,000AU.
Putting the figures into Kepler's equation