Simply enough, molarity is just the measure of the number of moles of a solute in liters of a solvent. So molarity (M) is equal to the number of moles of solute divided by the liters of solvent. M=n/V where n are moles of solute and V is the number of liters of solvent. There are three variables here. The problem has given you two of the variables, so you can solve it. 0.15M=n/0.250L so n = 0.0375moles of K_{2}Cr_{2}O_{7}.

I'm probably sure they'll want you to convert it into grams so just multiply it by the molar mass.