What s +=?

Answer 1

s = 5
s+= 3
means s = 8
in other words it's s = s + 3
you add the right veriable to the left
which means since our s = 8 now
doing s -= 4 will subtract 4 from s
so now s = 4

Answer 2

"x+=y" is the same that "x=x+y".

"+=" is a abbreviation of a sum where one term (x) will receive your value more the value of the "y".

Answer 3

Gotta say I'm impressed by how nice this community is. I came here and almost expected commentors ranting at the asker for not having read the explanation earlier in the lessons, but you go ahead and simply explain.