Alpha decay is the one that causes you to lose mass. Here we went from 238 in U to 218 in Po. Thats 20. Each alpha decay is -4 mass units. So 20/4 = 5 alpha decays.
So x = 5.
Now we are at 218 mass units. But alpha decay also loses -2 atomic number units. So U was atomic number 92. But we lost 2x5 (5 since we did 5 alpha decays) = 10 units from the atomic number. So we are at 218 Pb (which is atomic number 82).
But we need to get to 218 Po. Po is atomic number 84. So we need to change the atomic number by +2. We do a b- decay to change 1 neutron to a proton which causes the mass number to stay the same, but adds 1 to the atomic number. We do 2 beta decays (b-).
y=2.
