In the circuit shown in the figure, $R_{1}=3.00 \Omega_{1} R_{2}=6.00 \Omega, R_{3}=20.0 \Omega_{2}$
and $V_{\rightarrow n t}=12.0 \mathrm{~V}$.
a) Determine a value for the equivalent resistance.
b) Calculate the magnitude of the current flowing through $R_{3}$ on the top branch of the circuit (marked with a vertical arrow.