Two resistors 3 ohm and unknown resistor are connected in series across a 12 volt battery if the voltage drop across the unknown resistor is 6 volt find potential across 3 ohm resistance and the current through unknown resistor R and equivalent resistance of the circuit

Dear Student ,
Total resistance = 3 + 6 = 9ohm
V = IR
12 = 9 x I
12/9 = I
I = 1.33 A
a. V = ?
I = 1.33 A
R = 3 ohm
V = 3 x 1.33 = 3.99 volts

b. Current is the same in series 
that is 1.33 A

c. 9 ohms

Voltage drop
Total voltage - Voltage in 1st resistor 
= 12 - 3.99
= 8.01 ohms

Regards