Question 45262
15x2 – 21x + 6

This is called the "ac" method of factoring.
You want to find two numbers whose product is ac=(15)(6)=90
and whose sum is b=-21

The numbers are -15 and -6.
Rewrite the problem using -15x -6x in place of -21x, as follows:
15x^2-15x-6x+6

Now factor by grouping as follows:

15x(x-1)-6(x-1)
(x-1)(15x-6)

That's it.
Cheers,
Stan H.