Tip: Are you a non-native English speaker? I have just finished creating a 
Web AppI have heard a lot of people using the word “substract”, respectively its derived form “substraction”. Long story short, the correct forms are “subtract” and “subtraction” (without “s”).
The reason why so many English learners make the mistake is probably that there is a corresponding word in many languages that does contain an “s”, e.g. soustraire in French or sustraer in Spanish. Another reason might be that learners confuse its sound with other common words, e.g. “abstract”, “substrate”, “substitute”.
It is also worth noting that the form “substract” did exist in English until about the 18th century (due to French influence), but it is now considered completely obsolete.
