In an AP, the $\; 4^{th}\;$ term is $36$. The$\; 21^{st}\;$ term is $108$ more than the $\;9^{th}\;$ term. What is the first term a and the common difference $d$?

Question

$\begin{array}{1 1} a=15,d=12 \\ a=9,d=9 \\ a=5,d=10 \\ a=3,d=21 \end{array}$