Good Question! Notice that the 15 here is from the product of two numbers when multiplied would give 15. Also, these two numbers when added together give us -8. Thinking about this, we need to recall the factors of 15 include: 1, 3, 5 and 15.
At this point, I'm sure you're thinking of the two factors -3 and -5 which is a very good observation!
Therefore we can say the following:
( x - 3 ) ( x - 5 ) <=== FINAL ANSWER
* NOTE: As a check, you can use the FOIL Method on this binomial to get the Quadratic Expression that you were originally presented with! =)
I hope that helps you out! Please let me know if you have any other questions!
College Calculus Student ; Math Tutor