Calculus has nothing whatsoever to do with infinitesimals.
The entire point of calculus (and its more formal uncle Real Analysis) is to eliminate consideration of infinitesimals, replacing them with rigorous definitions of the real numbers and limits.
Did you learn about $$\epsilon - \delta$$? No?
Now, of course, calculus includes a lot more than limits but with "Quack" one needs to do one small step at a time.