How can I solve this exercise on Operational Amplifiers?
The second part of the solution of the exercise says, 


  \$\frac{R_2}{R_3} = \frac{R_1}{R_4} = 1\$


From this, its clear that they assume the circuit to be a unity differential amplifier. 

So it means, they missed out the term 'dBs' in the question. Differential gain , \$ G_d= 0  dB \implies G_d= 1\$.

It means find the relationship between \$R_1,R_2,R_3,R_4\$ for \$G_d = 0dB = 1\$

\$\implies V_{out} = V_d.G = V_d \$

\$\implies V_{out} = V_1-V_2\$

Comparing this with the Vout equation
$$V_{out} = V_1.\frac{R_1}{(R_1+R_2)}.(1+\frac{R_4}{R_3})-V_2.\frac{R_4}{R_3} $$

It gives

\$\frac{R_4}{R_3} = \frac{R_1}{R_2} = 1\$

Or

\$\frac{R_2}{R_3} = \frac{R_1}{R_4} = 1\$