Suppose that #q(x)# is the quocient and #n(x) = 6x^3+5x^2+2x+1, d(x)=2x+3,r(x)=c_0#. Then #q(x)d(x)+r(x)=n(x)# Putting #q(x) = ax^2+bx+c# (Here #q(x)# degree must be #3-1# because #d(x)# degree is #1#. By the same reason #r(x)# degree is #0#).
After multiplication of #q(x)d(x)+r(x)-n(x)=0# we get: #(2a-6)x^3+(3a+2b-5)x^2+(3b+2c-2)x+3c+c_0-1=0#. This equation must be identically null for all #x#. Solving for #a,b,c,c_0# we obtain: #q(x)=3x^2-2x+4, r(x)=-11#