How do you calculate the output voltage of an op-amp with DC voltage
In this case Opamp's high gain will drive both positive and negative inputs to same potential. it means that you have 0.75V over R1 or 7.5 mA. Same current will also flow thru R2 so 0.0075A*300R = 2.25V (ohms law)..

Output voltage is -Vin * R2/R1.