Answer:
l = 2 means d orbitals
electronic configuration of Fe2+ = [Ar] 3d6
six electrons in d orbitals 4 will be unpaired 2 will be paired
electronic configuration of Co2+ = [Ar] 3d7
three electrons will be unpaired four will be paierd
electronic configuration of Ni2+ = [Ar] 3d8
two electrons will be unpaired and six will be paired
So the sum of paired electrons is 2+4+6=12