The first and third have a leading coefficient of 1. So you know that the first terms have to be
To fill in the last terms, ask yourself, what are the factors of 20 whose sum is 9? It's +4 and +5, so fill them in:
The second and fourth are one of the special patterns that you should memorize -- these involve difference of two squares. The pattern is
a would correspond to s, and b would correspond to 10:
I'll let you try the other two.
01