The equation y=x^2-12x+45 models the number of books y sold in a bookstore x days after an award-winning author appeared at an autograph-signing reception. What was the first day that at least 100 copies of the book were sold?

Answer: The first day the author reaches 100 days is on day 16.

To solve this problem, you could use a graphing calculator to graph the given equation. Then, determine when this line crosses 100. It crosses when x = 15.539. Therefore, we would have to round up to 16 so it is at least 100.

You could use the quadratic equation to solve: 100 = x^2 -12x + 45

Either you will get 16. If you use the quadratic formula, make sure to only use the positive answer.