Solving this op-amp circuit
The answer is actually pretty simple : 

Voltage difference between \$ V_+\$ and \$ V_-\$ is zero. No current flows into the opamps input. Output voltage is the sum of opamp input voltage and voltage across resistor R2. Also, output voltage is dependent on R1.

*NOTE: Ix is the current source between the two input pins. If your source is 0.5Ios, simply substitute the Ix with 0 5Ios. Thanks goes to Andy_aka who caught my nomenclature mistake.*

Thus:

\$ V_+ = R_3 * I_{X}\$

\$ V_- = V_+\$

\$ I_1 = V_+\ / R_1 \$

\$ I_2 = I_1 + I_{X}\$

\$ V_{R2} = R_2 * I_2\$

\$ V_O = V_{R2} + V_{R3}\$

or

\$ V_O = I_{X} * (R_3 + R_2 + R_3 * R_2 / R_1)\$