Mathematics, Discrete Mathematics, Programming.
way, when you plug a number into an equation f(x) using a calculator, you should always get one unique y value for every unique x value put into f(x). If f(x) = 2(x) and x = 3 f(3) = 2(3) which always returns one output which is f(3) = 6. f(3) is always equal to 6 and will never be equal to another