Well, you have to be an idiot to make pH 5.5 buffer from phosphates (you are almost 2 units from pKa2, so buffer capacity is very low), but that's not your fault
You are close. You know concentrations. Calculate how much reagents will be present in the final solution, than calculate how much stock solutions contain these amounts of regents. Don't round down intermediate results. Fill up with water.
Note: don't use numbers from the attached picture, they are too exact for your needs - most likely they will be not accepted, as they take into account effects that your calculations ignore. It is just for your reference.