By superposition, mentally short out one of the signal voltages (thus keeping only the series resistance).
For each signal, the 'bottom' resistor is effectively 0.5R.
Therefore, we can sum the two separate currents, which combine in the "0.5R" effective bottom resistor.
I1 = 1/(1 + 0.5), I2 = 2/(1 + 0.5), I_SUM = (1+2)/(1.5) = 2
V+ = I.R = 2*0.5 = 1V
Now, The top part is simply a non-inverting amplifier, gain = 1 + (2/1) = 3
Finally, Vo = gain * V+ = 3*1 = 3V