U=RI=>R=U/I=6/0,25=24 Ω
U1=4 V=>I1=U1/R=4/24=0,16 A
I2=0,075 A=>U2=RI2=24*0,075=1,8 V
U3=12 V=>I3=U3/R=12/24=0,5 A