If a factor is x+4, that means that the coefficient of x^2 (the number in front of it, which would be 1) and that 20 should be divisible by 20. Now, if you divide 20 by 4, you would get 5, and 1/1 = 1. So, the 2 factors you have is
(x+4)(x+5)
When you FOIL, you would get
x^2+9x+20, which means
b=9