Start with several of these, with x's as "the firsts"
since x times x is x², the first term.
(x )(x )
(x )(x )
(x )(x )
Think of the ways to factor 20 using two factors, 1·20, 2·10, 4·5
put those in for "the lasts"
(x 1)(x 20)
(x 2)(x 10)
(x 4)(x 5)
Put +'s between because everything has a +. Then FOIL them out
(x + 1)(x + 20) = x² + 21x +20
(x + 2)(x + 10) = x² + 12x + 20
(x + 4)(x + 5) = x² + 9x + 20
So the middle term could be 21x, 12x or 9x
Edwin