Your answer is wrong. You are losing 250 microamps of current from the 2.5 volts going through R3. That has to be made up from extra current present in R4. That will increase the voltage across R4 by 3.75 volts, making the correct answer 10 - 1.5 Vin. You can also calculate the correct answer by Thevenizing the input circuit and doing the calculation from the Thevenin resistance and open circuit voltage.
