In this ideal op-amp circuit, it is given: $R_1 = 6$ ohms, $R_2 = 5$ ohms, $R_3 = 5$ ohms, $R_4 = 78$ ohms, $R_5 = 9$ ohms, and $R_6 = 64$ ohms.
What is the voltage gain $G = v_o / v_s$ ? (Hint: first replace $v_s$, $R_1$, $R_2$, and $R_3$ with their Thevenin equivalent circuit)