9.

 

x^2+10x+21

When we multiply the first and last coefficients, (1)(21) = 21 so

we are looking for a pair of factors of 21 which add up to 10. There is

such a pair, 3 and 7. This gives us the coefficients of the O and I

terms when we recover the FOIL step.

 

x^2+7x+3x+21

 

This comes from

 

(x+3)(x+7)