Help! How do I rearrange equations?

Being able to manipulate equations is a very useful skill in Physics. Consider, for example, the relationship between mass, density and volume.
m = ρ x v
In the above equation, the mass, m is the subject because it appears all on it's own on one side of the equation and does not appear on the other side. If we wanted, for example, to make the distance ρ the subject and to get it all on its own, we should ask ourselves: What mathematical operation is being applied to ρ? We can see that it is being multiplied by the volume v, so to get ρ alone we must divide it by v. Now comes the crucial rule when it comes to algebra, an equation remains unchanged as long as you do the same thing to both sides. Therefore for the equation to still make sense, we must also divide the mass by v and then we equation becomes
m / v = ρ