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