Integer arithmetic extends ordinary counting-number arithmetic to include negative numbers and zero, and one specific rule trips people up more than any other: subtracting a negative number turns out to be exactly the same as adding its positive counterpart. That rule isn't an arbitrary convention someone decided on for convenience, it's a direct, forced consequence of what subtraction and negation each already mean once negative numbers are allowed into the picture at all.
The number line gives a genuine visual model for negative numbers and basic operations
Addition moves rightward along the number line, subtraction moves leftward, and negative numbers simply occupy the other side of zero from the familiar positive counting numbers. Multiplication and division's sign rules, same signs produce a positive result, different signs produce a negative one, follow directly from extending the pattern that already holds among positive numbers consistently into negative territory, rather than being a separate, unrelated rule bolted on afterward.
Subtracting a negative equals adding a positive because subtraction is defined as adding the opposite
Subtraction is formally defined as adding a number's opposite, a minus b is the same as a plus the opposite of b. Once b itself is a negative number, its opposite is a positive number, negating a negative value flips it back to positive, which means subtracting a negative number becomes, by that same definition, adding a positive one. It isn't an arbitrary rule invented separately for negative numbers, it's the same subtraction-as-adding-the-opposite definition already in use for positive numbers, simply carried through consistently once negative numbers are allowed in.
What we're still unsure about
That subtracting a negative number equals adding its positive counterpart, and that this follows directly from subtraction's own definition, is well established, uncontroversial arithmetic. What's more genuinely a studied, unresolved question in mathematics education is that many learners can correctly apply the double-negative rule mechanically without actually understanding why it has to be true, being able to execute a rule correctly and being able to explain why it's necessarily true are measurably different skills, and which one actually predicts a student's later success in algebra is a genuinely researched, not yet fully settled, question.
This sits inside Integer Arithmetic, one of seven topics in Arithmetic, one of seven domains in Mathematics, one of seventeen subjects the app can quiz you on.