I couldn't figure out how to rearrange so i looked at the answer but it still didnt help.

If V = 12R / (r + R) , then R =

Vr / (12 - V)

We have to rearrange the equation to make R the subject.

Start by cross multiplying by (r + R); V (r + R) = 12R

Multiply out the bracket Vr + VR = 12R

I pulled this off a website, I can't figure out how they get to Vr + VR=12R

but I cant figure out how to get from that to Vr/(12-V)