Suppose we have an expression such as Q(x) = x^2 + 4x - 9. Q tells us the action performed, so to speak, on the variable x. Here, Q tells us to square x, multiply it by 4, and then subtract it by 9. So for example, if we have a function that reads Q(7), we square 7, which gives us 49, and we multiply 7 by 2, which gives us 14, and then we add 49 + 17, which gives us 66, and then we subtract this 66 by 9, which gives us 57. Note that all we have done is input the number provided for the variable x in the original functional expression, and it is this process which gives us our final answer of 57. Simply input the definition, so to speak, given on the left side of the equal sign, and we get our output, which is provided on the right side of the equal sign.