In this section of a circuit, resistors R1, R2, and R3 combine for an equivalent resistance of 4.0 ohm. What is R2? Let R1 = 6.0 ohm and R3= 2.0 ohm.

Answer:
R2=10ohm
Explanation:
In this combination
R2 and R3 are in series combination where R1 is in parallel combination with them hence
R2+R3=R`
And
1/R`+1/R1=1/Req
1/R`=1/Req-1/R1
1/R`=1/4-1/6
=1/12
R`=12ohm
Also
R2+R3=R`
R2=R`-R3
R2=12-2
R2=10ohm