Astronomers using the Hubble Space Telescope have recently deduced the presence of an extremely massive core in the distant galaxy M87, so dense that it could well be a black hole (from which no light escapes). They measured the speed of gas clouds orbiting the core to be 758 km/s at a distance 5.58 x 1017 m from the core. Calculate the mass of the core. I'm at a complete loss as to how to calculate the mass because I have no radius to work with. What alternative is there to m = gr^2 / G?