For carbon, #Z=6#, and thus there are #6# positive nuclear charges.
For methane, #CH_4#, there are #2# inner shell electrons i.e. (#1s#), and 8 electrons in the #4xxC-H# bonds. Carbon has a half share of these, and thus the carbon atom is presumed to be associated with #6# electrons that precisely balance the #Z=6# nuclear charge to give a formally neutral carbon.
For a carbocation, #R_3C^+#, there are only 5 electrons associated with the carbon; and hence the carbon bears a formal positive charge.
Let's try another question. Dichlorcarbene #Cl_2C:# is not something you could put in a bottle; it is a species with which you could do a reaction. Can you tell me the formal charge of #"dichlorcarbene"#?
