We perform the synthetic division
#color(white)(aaaa)##-3##color(white)(aaaa)##|##color(white)(aaaa)##6##color(white)(aaaa)##28##color(white)(aaaaa)##25##color(white)(aaa)##-21#
#color(white)(aaaaaa)##color(white)(aaaaa)##|##color(white)(aaaaa)##color(white)(aa)##-18##color(white)(aaaa)##-30##color(white)(aaaa)##15#
#color(white)(aaaaaaaaaa)#------------------------------------------------------------
#color(white)(aaaa)##color(white)(aaaaaa)##color(white)(aaaaaa)##6##color(white)(aaaa)##10##color(white)(aaaa)##-5##color(white)(aaaa)##color(red)(-6)#
The remainder is #=-6#
The quotient is #=6b^2+10b-5#