find voltage?

Ohm's law:
V = IR   ⇒   I = V/R

Refer to the figure below.

Voltage across a and c, Vac
= (2 A) × (Req)
= 2 × 2.5 V
= 5 V

Current I₁
= Vac/(5 Ω)
= 5/5 A
= 1 A

Current I₂
= (2 A) - I₁
= (2 - 1) A
= 1 A

Voltage across a and b, Vab
= I₂ × (3 Ω)
= 1 × 3 V
= 3 V

Voltage across b and c, ν
= Vac - Vab
= (5 - 3) V
= 2 V

The answer: (b) 2 V

From the given current and resistance, use Ohm's law to calculate the voltage across the whole network.  Now you can work out the current flowing through the 5 ohm resistor.  The current through the rest of the network is 2 amp minus that.  Since that current passes through the 3 ohm resistor, you can calculate the voltage drop across that.  Deduct that from the total voltage to give your answer.

Hint: The fact that this voltage drop is shown across two resistors in parallel is not actually relevant.  It could be a single resistor.

(3+x) // 5 = 2.5
(3+x)*5 / (3+x+5) = 2.5
15+5x = 2.5x+20
2.5x = 5
x = 5/2.5 = 2.0 ohm 
3+x = 3+2 = 5 ohm 
V = I/2*x = 2/2*2 = 2.0 V