Please bare with me, i'm not sure how to put in the maths on a forum like this. Ok so i was asked to combine two equations to get a third, this I have done. The equation is M = v squared x r and then the producet of those divided by G. M is the mass of a black hole v squared is the velocity of an object orbitting the black hole r is the radius of the object from the centre of the black hole G is newtons universal constant of gravitation given as 6.7 x 10 to the minus 11. 3. The attempt at a solution The maths I don't need help with, i'm sure I have that right, it's the units i'm struggling to understand. v is given as Km s-1 r is given simply as Km G is given as 1 m3 kg-1 s-2 This would give units of (Km s-1)2 x Km m3 kg-1 s-2 I'm sure Kg is the actual unit I should end up with but i don't see how i get there with thee units. Again I apologise for not being able to enter the maths a little better, hopefully i've explained it well enough.