How do you calculate Input output voltages of a trans-resistance op amp?
Your first calculation is correct in the sense that 300uA x 10 kohm = 3 V but that voltage is across R1, you assume Vn = 0 V which it isn't.

So 3 V across R1, note the direction of the current flow, so Vn - Vo = 3 V

We still do not know Vn or Vo yet !

Indeed R2, R3 is a voltage divider, your formula is correct.

Since we assume the opamp to operate in feedback correctly, we can assume Vp = Vn

Conveniently, since Vp = Vn we know that the voltage across R3 is the same as the voltage across R1 so also 3 V.

Conveniently R3 is 30 kohm and R2 is 20 kohm. The current is the same (it cannot go into the opamp) so the voltage across R2 (20 kohm) is 2 Volt.

The other side of R2 is grounded to Vp is 2 V, now you solve the rest !

For (b) you again assume Vn = 0 and that is not true ! To solve (b) you will have to write down all the equations and solve them.

You could also replace I1 and the extra resistor by its Thevenin equivalent if that helps.