The concentration of the target solution tells you how much solute, in your case hemoglobin, is needed per milliter of solution.
In this case, you know that the target solution must have a concentration of
If you weigh out
#95color(red)(cancel(color(black)("mg"))) * "1 mL solution"/(15color(red)(cancel(color(black)("mg")))) = "6.333 mL"#
of phosphate-buffered saline, PBS, to make the target solution.
You need to round this value to two sig figs, the number of sig figs you have for the concentration of the target solution and for the mass of hemoglobin
#V_"PBS" = color(green)("6.3 mL")#