Question 59663
When 16b^2 + 10b - 21 is factored, one factor is: 

 4b - 7 
 8b + 7 
 2b - 3 
 16b + 3 
 (none of these) <---answer
:
16b^2+10b-21  This is the ac method.  a*c=16*-21=-336  replace b, (10) with two numbers that multiply to give you -336, but add to give you 10.  I used the prime factors of 336 to find them.
16b^2+24b-14b-21
(16b^2+24b)+(-14b-21)
8b(2b+3)-7(2b+3)
(2b+3)(8b-7)  <---Neither of these factors is a choice.
Happy Calculating!!!