@Shahrina86 thank you the problem has been solved. 

@Carl Love thank you so much. it worked in giving the backward loop. And the n-2 is correct, because b_{n-1} is given by a_n (second last value), while b_n=0 (last value). The looping should start from the third last value. By the way, when I run the coding, it yields b_2, b_1, and b_0. It did not provide the values of the coefficients. Should I use the solve command here? Suppose, b_5=0, b_4=0, b_3=0, b_2=0, b_1=1, b_0=2c.