The concentration of PNP is 180 µmol/L.
We have to assume that the volume does not change when you add the PNP to the NaOH solution. If the volume changed by ±1.25 µL, the change would be ±1 %. You should check with your instructor if this assumption is allowed.
Step 1. Calculate the concentration of PNP before dilution.
Step 2. Calculate the concentration of PNP after dilution.
You added 12.0 µL to 488 µL, so your new volume was 500 µL. Now we do a simple dilution calculation.