A black hole technically has no size, so the gravity at its heart would be infinite (r=0). But before you get to the center, you leave the Universe at the event horizon of the black hole. To find the value of r for this event horizon, you use this: r=(2GM)/c^2, where c is the speed of light = (3.00 x 10^8 m/s); and M is the mass of the black hole. Use this value of r to find the "surface" gravity (g) for a black hole of 12 times the mass of the sun.