it appears that this is a step in factoring
in order to find the two numbers that add to -17
we need to first find all the factors that multiply to 21
to add to -17, ll the factors of 21 will be negative
21
-1, -21
-3, -7
-7, -3
-21, -1
we can try and add the two pairs of each number together
but none of the combinations will work.