Network Identification - How to find R1 and R2
The symbol with the diamond box and arrow indicates a current sink, labelled as \$ i/2 \$.
Therefore half of the input current in the circuit will flow through this branch.

Applying Thevenin's theorem we know that with the current source switched off half the input current will flow through the resistor \$ R_1 \$ and all of the current will flow through \$ R_2 \$. As we are trying to find the equivalent resistance for the full circuit (\$ R_{eq} = V/I_{in} \$) therefore as you noticed, the resistors are in series and so \$ V = i_{R_2} \cdot R_1 + i_{R_2} \cdot R_2 \$.

Hope this helps,
Alex