Adding integers is something to pick up since you will use it in all math. So for this problem we have 3 electrons which are negative (-red dots) and we have 3 protons which are positive (+ blue dots). When we add them this happens:
Proton + electrons = ?
3+ (-3)=0
0 is neutral, meaning not negative or positive so that means your lithium atom has neutral charge.
If you had a number line and you put your finger on 3 you would move 3 spaces left towards the negative numbers because 3 is negative and it would land you at 0.