c=(m-1) /2 right?
Die Ruhe vor dem Sturm.
To make c the subject you need to get rid of the "+1" and the "*2"

So to do that, you gotta subtract one and divide by two.

And whatever you do to one side, you do to the other

So it's m-1 = 2c

and (m-1)/2 = c

Get it?
Because what you do to one side you do to the other. You want to get C on its own, so first you have to get rid of the +1. So you take 1 from each side, getting (m-1)=2c.

You then want to get rid of the 2, so you divide 2c by 2. You have to do this to the other side as well, so you get (m-1)/2=c.
just move m to the predicate

VOX AC15 / AC4
Fender MIM Sunburst Strat
epiphone les paul custom in white
Yamaha FG730s
The Tweak-O fuzz pedal