#(r^3+6r^2-21r-18)-:(r-3)#
#color(white)(aaaa)##3|#######
#color(white)(aaaaaaaaaaa)##---------#
#color(white)(aaaaaaaaaaaa)#1#color(white)(aaaa)#6#color(white)
(aaaa)#-21#color(white)(aaaa)#-18
#color(white)(aaaaaaaaaaaa)##color(white)(aaaaa)#3#color(white)
(aaaa)#27#color(white)(aaaa)#18
#color(white)(aaaaaaaaaaa)##---------#
#color(white)(aaaaaaaaaaaa)#1#color(white)(aaaa)#9#color(white)
(aaaa)#6#color(white)(aaaaaa)#0=remainder
#color(white)(aaaaaaaaaaa)##---------#
#r-3=0#
#r=3#
#:.#= # r^2+9r+6#