The dilution factor will be related to how the calibration curve was constructed. If you constructed the curve using solutions with solute concentrations between 0 and 50 ng/ml, and the sample you prepared is not further diluted then the concentration in your sample is 

conc (ng/mL) = concentration of standard / area of standard * area of sample

To change this to ng/mg then: conc (ng/mL) * 0.15 mL / 5

Thanks OMAR

i would like to inform you in my case i am not using calibration curve that's why i was asking about using Dilution factor here

It is not necessary if you do not dilute your sample further

Thanks OMAR.