Do you mean:

\frac{z}{y+r} = rw

And you need to make which variable the subject of formula? r?

Yes but I do not understand on how to solve the problem

It's simple as soon as you understand what you have to do.

Since you have 2 r's on different sides of the equal sign, one being in a fraction, you 'cross-multiply' to get this:

z = rw(y+r)

Now you expand:

z = rwy + r^2w

Now, this is getting a little difficult if you don't know the quadratic formula... (or completing the square) which goes when you have this:

r^2w + rwy - z = 0

Can you tell use which of the two (quadratic formula or completing the square) you know how to use?

Ohh I need a class of math & chemistry lol

Thanks

Uh... no problem. I assume that you solved it now?