α - particle is represented by He atom with mass number of 4 and atomic number of 2. Thus, try to balance the mass number and atomic number of both Ra and Rn by subtracting 4 from 226 i.e (226 - 4) and subtracting 2 from 88 i.e (88 - 2 = 86 ).

\(^{226}\)\(_{88}\)Ra → \(^{222}\)\(_{86}\)Rn + \(^4\)\(_2\)He (α - particle)