Calculating resistance in a circuit where relay is controlled by microcontroller
You don't need to use the 12V supply voltage for anything. It's not uncommon for questions to include some extraneous information. 

If you assume the input to be a (theoretical) value the same as the MCU supply voltage it might be 5.0, 3.3, 2.5, 1.8 or something else entirely. A reasonable assumption might be 5.0, but you should state your assumption clearly. 

In reality you would want to use a much lower value for the resistor than the value you will calculate above using Ohm's law and the nominal Vbe drop, to ensure deep saturation for the transistor and to account for the voltage drop in the MCU output transistor, voltage tolerances, temperature effects on the transistor, MCU output circuitry and relay coil etc.