In a function, you need to substitute your input value for x in order to get your output. For example, if you have the function f(x)=3x, and your input value is 3, you have to substitute in 3 for x. In this case, 3 x 3 is equal to 9, therefore, your output for 3 in this function is 9. Keep in mind that the output always changes between equations.
