A function can be written as `f(x)`, which is shorthand for the function of `x` is such that followed by an expression for the function eg `f(x) = 2x + 4`. The right hand side states what will be done to `x`: multiply by 2 and add 4.

A function is written with a single letter (normally `f`) followed by a variable (normally `x`) in brackets on the left hand side of the equation sign; and an expression (the function to be applied to that variable) on the right hand side.

When a function is evaluated, replace the variable with the number: using the above function,

`f(3) = 2 xx 3 + 4 = 10` or

`f(-5) = 2 xx -5 + 4 = -6`

Sometimes the function may be written slightly differently with a colon and an arrow:

`x: → 2x + 4` or even as `x ↦ 2x + 4`.