Answer
$^{214}_{84}Po\rightarrow\,\, ^{210}_{82}Pb+\,^{4}_{2}He$
Work Step by Step
Let us write the incomplete nuclear equation:
$^{214}_{84}Po\rightarrow\,\, ^{A}_{Z}X+\,^{4}_{2}He$
As the sum of the mass numbers on both sides should be equal, we have
$A+4=214$ or $A=210$.
As the sum of the atomic numbers on both sides should be equal, we have
$Z+2=84$ or $Z=82$. The element with Z=82 is Pb.
Therefore, the complete nuclear equation is
$^{214}_{84}Po\rightarrow\,\, ^{210}_{82}Pb+\,^{4}_{2}He$
