Evaluating Functions

The concept of FUNCTION is an important one in algebra. In the equation y=x², the value of y is determined by the value of x, so we say that y is a function of x. For any given value of x, we can find y by simply plugging x's value into the right hand side of the equation wherever x appears. When y is a function of x, we often show "f(x)" on the left side of the equation in place of y.
 
Let's consider the following function:

f(x) = 9x2 + 8x - 5

What is f(x) at x=4?
 
To evaluate this function, we simply replace every x with 4:

f(4) = 9 * (4)2 + 8 * (4) - 5 = 171

In the function below, what is f(x) at x = -7?

f(x) = 6x2 - 5x + 1

First replace every instance of x with -7:

f(x) = 6 * (-7)2 - 5(-7) + 1

Now simplify and solve:

f(x) = 6 * 49 + 35 + 1 = 294 + 35 + 1 = 330