What operation undo each other?

Answer 1

Inverse oprations.

Here are some examples (with some values excluded where one or the other operation is not defined or where one of the functions is not uniquely defined):

Addition and subtraction are inverses of each other,

Multiplication and division are inverses of each other,

Exponentiation and logariths are mutual inverses,

Trigonometric functions and their arc equivalents are mutual inverses,

Clockwise rotation and anticlockwise rotation are mutual inverses.

Squaring (a non-negative number) and the principal square-root of a non-negative number.