There is more than one way to do this but I think the simplest is to recognize that the 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 levels are full. If you count that up it adds to 36. I would, with my handy dandy calculator add the electrons shown (start with 36) and look up the atomic number on the periodic table for total electrons.
