When there is a voltage source as here (0 and 5 V), then for analysis purposes you connect them together, as a perfect voltage source has a 0 Ohm impedance. So once you do that, it becomes a simple combination of two parallel resistors. For two resitors, R1 and R2, the combination is R1*R2/(R1+R2). For n resistors in parallel, one uses the formula 1/R=1/R1 + 1/R2 + 1/R3 .... 1/Rn.
Someone else has already worked it out for you, but I thought I'd explain why his/her method is correct.
Dave