The units of molarity are mol/L, so first we need to convert 900 mL to liters (using the conversion that 1000 mL = 1L). Next, multiply by the molarity (6.1 x 10-2 mol/L) to give the amount of moles of KMnO4 needed. Finally, multiply by the molecular weight of KMnO4 (158.034 g/mol).

900 mL (1 L / 1000 mL) (6.1 x 10-2 mol/L)(158.034 g / mol) = 8.67 g KMnO4

A quick dimensional analysis shows that all the units cancel except for g of KMnO4