This is pretty simple actually :) You are going to use the equation M1xV1=M2xV2
M stands for molarity and V is volume. Plug in the information you have, and then solve for the unknown using simple algebra. You get 6.0M NaNo3 x 250mL= 15M NaNO3 x V2. Now solve for V2. Keep in mind that you will have to use stoichiometry to convert to liters instead of liters if your answer requires it. Hope this helps! :)