Thankyou, I was under the impression I could work out the gain = Vout/ Vin

If this were the usual op-amp circuit, with non-inverting gain for a single input applied to the (+) input, then you could say that the "gain" for that input is 1 + R

_{f} / R

_{a}. You should derive this equation yourself to prove you know what you are doing.

However, it becomes a little more complicated when the output voltage and the input voltages are all specified but the resistor values are not. Do what

@Harald Kapp said: calculate the voltage at V

_{p}. This will be the same as the voltage at V

_{n}. From this information you should be able to calculate resistor values for R, R

_{f} and R

_{a} to "make it so" to produce 5V output with the three specified input voltages. Note that the resistor values are arbitrary in their absolute values.