Find voltage across inductor in RL circuit
If I told you that there is 5 mA flowing through the inductor just before the switch opens can you see that? It’s based on the assumption that the switch has been closed for a long time. Do you understand that bit?

Can you also see that as the switch opens, in that instant there will still be 5 mA flowing in the inductor?

Based on that, you can calculate the voltages across R2 and R3 and the current source. The voltage across the current source is defined, in that instant, by 5 mA also flowing through R1 and R4.

Can you take it from here?