Go to frame 7:50
The man explains why it shouldn't be -q
unfortunately I am too dumb to fully understand.
Let the charge on the most left side be Q1, the middle charge be Q2, and the chargeon the right be Q3
The question wanted to ask what the force on Q2 be, the total force.
Now he explained something about "it's already taken care of"
F2 = F23 + F13
Is the man trying to say that F23 is negative because it is pushing left?
I pulled that formula out of my book. The way he did it is F32 because it is to the right