The equivalent resistance of three resistors in series is given by the sum of the resistances:
[tex]R_{eq}=R_1+R_2+R_3[/tex]
In this circuit, the value of the three resistances is:
[tex]R_1=3 \Omega[/tex]
[tex]R_2=4 \Omega[/tex]
[tex]R_3=5 \Omega[/tex]
Therefore, the equivalent resistance of the circuit is:
[tex]R_{eq}=3 \Omega+4 \Omega + 5 \Omega =12 \Omega[/tex]
