Answer:
Carbon has 6 electrons.
Before I answer, here's what you need to know.
There are 7 shells. Each shell can hold a certain amount of electrons.
1: 2
2: 8
3: 18
4: 32
5: 32
6: 18
7: 8
Carbon has 6 electrons, remember? So shell 1 is already completely filled. Shell 1 completely full, so onto shell 2.
Shell 2 needs 8 electrons, but carbon only has 4 to give, so 2, 4.
I hope this helps!