To balance nuclear equations recognize that the top number is the mass and the bottom number the atomic number (# of protons). The total mass on both sides must be the same and the total # of protons on both sides must be the same.
On the left you have a total mass of 231 and a total of 91 protons.
On the right you have an alpha particle which is Helium with a mass of 4 and 2 protons.
91 -2 = 89 protons for the decay product which is element Ac, actinium.
231 - 4 = 227.
So the decay products are helium 2 4 and Actinium with a mass of 227.