How to calculate voltage summing op amp
There are many ways to solve. One way of solving is:

Voltage at node b = 0.25 V (Voltage Divider)
It should be equal to the voltage at node a too, in ideal op-amp.
Hence
$$V_a = V_b = 0.25 V$$
Now current through 5k resistor would be 
$$(V_2 - V_a)/5k = 150 uA$$
The same current flows through the 10k feed back resistor. Because, from node a, no current flows into the -ve terminal of the ideal op-amp. Therefore:
$$(V_a - V _o)/10k = 150 uA$$
On solving:
$$V_o = - 1.25 V$$