In the given circuit if I1 and I2 are the currents in resistors of resistances R1 and R2, respectively, then

Since 3 A is in upper part of circuit. Therefore out of 5 A coming in lower part, 3 A has to go to the upper pert, out of which some part will flow through R2 and rest through the unknown resistance.∴ 2 A will go through R1.