The three forces (in units of n) given below are acting on a 20 kg mass. calculate the magnitude of the acceleration of the mass. stack f subscript 1 with rightwards harpoon with barb upwards on top equals 3 i with hat on top space space space space space stack f subscript 2 with rightwards harpoon with barb upwards on top equals 5 j with hat on top space space space space stack f subscript 3 with rightwards harpoon with barb upwards on top equals open parentheses i with hat on top minus 3 j with hat on top close parentheses space space