add, subtract
multiply, divide
earth quakes
The opposite of a number is another number (possibly the same), such that when the two numbers are combined according to some operation, the result is the identity for that operation. The opposite of a number will, therefore, depend on the operation. Addition (identity = 0) and multiplication (identity = 1) are two common operations.
Addition is the inverse operation of subtraction and multiplication is the inverse operation of division. The word inverse means "opposite".
The last two operations in P.E.M.D.A.S are 'addition' and 'subtraction'.
You would certainly use an op-amp or an operational amplifier, if two opposite sides which successfully met and made operations correctly with no bugs.