I made some assumptions, but here's how to do it.
Step 1. Calculate the concentration of the original protein
This comes from Beer's Law:
I will assume that your protein had
Step 2. Calculate the concentration of the diluted protein
You took 15.0 µL of the sample and diluted it with 15.0 µL of buffer to get 30.0 µL of solution.
You get the new concentration from a standard dilution calculation.
Step 3. Calculate the mass of protein loaded on the gel
You loaded 20.0 µL of the dilute solution onto the gel.