Rewrite the expression in factored form: x2-11x+30

(X-5) (X-6)

First you look at the last number and find multiple of 30 that also add/subtract to get 11.

In this case 5 and 6 both "add up" to 11 and multiply to get 30.

BUT since you're looking for a negative 11 you can't add positive 5 and 6 so that is why you add together negative 5 and 6.

And in multiplication rules two negatives make a positive.