9. Factor.

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.

F O I L

*x*^{2} + 7*x* + 3*x* + 4

This comes from