According to Ohms Law the current through a resistor is proportional to the voltage acropss it, ie. Current (I) = V / R.
So if you have a supply voltage of 10V, the current through the 1000ohms resistor= .01A. And the current through the 3000ohm resistor would be .00333A. That makes the current through both resistors in parallel = .01A + .00333A = .01333A.
Again, from Ohms Law, if 10V produces a current of .01333A through the total resistance, the resistance must be 10/.0133A = 750ohms.
I hope that explains it.