Because I flow through the resistor U 12, the voltage U 12 = the voltage U3 on the resistor 3ω+the voltage U 18ω.

U 12=Ix 12，U3=(I 1-I)x3，u 18 =(I 1-I+I2)x 18

12I = 6x 3-3I+ 18(6-I-2)= 18+72-2 1I = 90-2 1I

33I=90

I=30/ 1 1A