Ohm's Law : V=Z.I . It is one equation with three variables. You got to have at least two to calculate the third. Makes sens, right ?
So, for any given V and I, you'll have your Z. For any given V and Z, you'll have your I... etc, for any given PAIR of known values, you'll have the third.
What do you have here ? The V0= 5volts. V1=2 volts. V2=3volts. V0=V1+V2. Anyway :
V1=R1.I ... 1
V2=R2.I ... 2
Either:
a- You got V1 so you SET arbitrarly R1 (to a normalized value if it's a board you entend to really make), you could then get the I, inject it in 2 and get R2.
(Same thing can be used in 2, you set R2, get I and inject it in 1)
b- You set arbitrarly I, and inject it in both 1 and 2 and get R1 and R2.
But the point being that, you always look at what you have, at the equations. V=Z.I here. It takes TWO to tango.