1. make p the subject:

Question

p/p-3= R

2. make R the subject

a= 1- 1/R

Answer 1

You must mean
p/(p-3) = R
cross-multiply
Rp - 3R = p
Rp - p = 3R
p(R-1) = 3R
p = 3R/(R-1)

for the second, multiply each term by R, rearrange and factor in a similar way.
Let me know what you get.

Answer 2

So for the second one, I multiplied each term by R

aR= R- 1

but then I got stuck after that... sorry

Answer 3

you are looking for R, so get the R's to one side

aR - R = -1
factor out the R
R(a-1) = -1
divide both sides by a-1

R = -1/(a-1)