you need to have the exact amount of atomic number and mass number on each side of the arrow. on the left of the equation Ra has 226 mass and 88 atomic. on the right the He with the atomic number of 2 and the mass number of 4. you’d minus the atomic number on the left with the atomic number on the right (88-2) getting 86 and minus the mass number on the left with the mass number on the right (226-2) which is 224
this means that A = 224 and B = 86
