Question

t=p+3r/6 solve for r

Answer 1

it's not that difficult, you're just making r the subject in terms of t and p

Answer 2

did you get it?
so you have:
t-p = 3r/6
6(t-p)=3r
so
r= 6(t-p)/3

which can be simplified to 2(t-p) = r