This is an excerpt from my math models textbook. It's about Lagrange Polynomials which is a technique that lets you fit a polynomial to a set of any number of unique points (x_1,y_1) ... (x_n,y_n) so long as all your x-values are different (otherwise it wouldn't be a function, and couldn't be a polynomial). The polynomial you'll calculate will be the unique, lowest degree polynomial that passes through all points.
The function should be cubic, so you should be able to write it in the form "f(x) = ax^3 + bx^2 + cx + d". You could work out the entire thing to put it in that form, but you don't need to.
Since there are no weird operations, roots, divisions by x, or anything like that, you can just count how many times x might get multiplied with itself. At the top of each division, there are 3 terms with x, so you can quite easily see that the maximum will be x^3.
It's useful to know what the values x_i and x_y are though. They describe the 3 points through which the function should go: (x_1, y_1) to (x_3, y_3).
That also makes the second part of the statement ready to check. Take (x_1, y_1) for example. You want to be sure that f(x_1) = y_1. If you replace all of the "x" in the formula by x_1, you'll see that everything starts cancelling each other out. Eventually you'll get "1 * y_1 + 0 * y_2 + 0 * y_3", thus f(x_1) is indeed y_1.
They could have explained this a bit better in the book, it also took me a little while to figure it out.