first of all a 3 ohm resistance is connected in parallel with 6 ohm resistance so we will have
[tex]\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2}[/tex]
[tex]\frac{1}{R} = \frac{1}{3} + \frac{1}{6}[/tex]
[tex]R = 2 ohm[/tex]
now it is connected in series with 4 ohm resistance
So we will have net resistance given by
[tex]R_{net} = R_1 + R_2[/tex]
[tex]R_{net} = 2 + 4 = 6 ohm[/tex]
Now this combination of 6 ohm resistance is connected across a battery of 12 V
so now we will have total current in the circuit calculated by ohm's law
[tex]V = i*R[/tex]
[tex]12 = i*6[/tex]
[tex]i = 2 A[/tex]
now this 2 A current will divide in 3 ohm and 6 ohm resistance in the ratio of 2:1
so current in 3 ohm resistance is given by
[tex]i = \frac{2}{2+1}*2= 1.33 A[/tex]
now power dissipated in 3 ohm resistance is given by
[tex]P = i^2 * R[/tex]
[tex]P = (1.33)^2* 3[/tex]
[tex]P = 5.33 Watt[/tex]