In a nuclear equation, the nucleus before the decay is on the left and the products are on the right. Each particle is written with its mass number at the top and its atomic number at the bottom. An alpha particle is 42He, a beta-minus particle (an electron) is 0-1e and a positron is 0+1e.
An equation is balanced when the total mass number on the left equals the total on the right, and the total atomic number on the left equals the total on the right. The atomic number is the charge of the nucleus, so balancing atomic numbers also balances charge.
For example, uranium-238 (atomic number 92) emits an alpha particle: 23892U → 23490Th + 42He. The mass numbers balance (238 = 234 + 4) and so do the atomic numbers (92 = 90 + 2). In beta-minus decay, carbon-14 becomes nitrogen-14: 146C → 147N + 0-1e, because 6 = 7 + (-1).
For a positron, sodium-22 (atomic number 11) becomes neon-22: 2211Na → 2210Ne + 0+1e, because 11 = 10 + 1. To find a missing number, subtract the known numbers on one side from the total on the other.