It means that you have a 1k ohm resistor in parallel with a 2k ohm resistor.
To calculate parallel resistors use 1/R=1/R1+1/R2+ ... +1/Rn.
For just two resistors you can use R=R1*R2/(R1+R2).
So you equation reads 15/(2000 + (1000*2000)/(1000 + 2000))=0.005625
Hope this helps.