well you know the banner is 11 yards and derek cut off 30 feet. so first lets convert the measurements to the same units.
11yards at 3ft=1 yard is 33ft
so the banner initially is 33ft
derek cut 30 ft
33ft-30ft= 3ft
so there is 3 ft left
need to see if theres at least 18in left
convert to same unit
3ft at 12in=1ft is 36in
36in-18in= 18in
so yes there will be enough with 18in still left