All the information is given in the expression:
Find factors of #15 and 28# whose products differ (because of #-28#)
by #1# (because of the #-1n# term.)
The signs will be different #(-28)# and there should be more negatives, (because of the #-1n# term.)
NOte that #" "-1n" "# is odd, which must come from the difference between an odd and an even number.
Therefore #2 xx 14# are not possible as factors of #28# because they will give even products.
#15 and -28#
#darr" "darr#
#3" "+4" "rarr 5 xx +4 = +20" "larr# even
#5" "-7" "rarr 3 xx -7 = -ul21" "larr# odd
#color(white)(xxxxxwwwwwwwwwwwwxxxx)-1#
The factors are #(3n+4)(5n-7)#
