A current source can have any voltage you choose around it. A current source has infinite impedance.This is a strange exercise! Who did it?
The voltage over the 4 ohm resistor must be 24 V.
So the the current -I2 is 24/4=6 A.
Why is that? A current source cannot have voltage, i.e it is zero.
In practical life this is not possible.
If the target is to teach circuit analysis, I dont know if it is a good idea
with such examples.