Working out the original concentration?

I'm trying to determine the amount of phosphate in a solution. 0.2 mL of test solution is added to a test tube, and reagents are added to a final volume of 3.0 mL and standard curve is created with all tubes having 3.0 mL final volume. After reading from the standard curve, the test sample is found to contain 0.5 mol phosphate. How do I figure out what was the concentration of phosphate in the original test solution is in mol/L?