In expansion of $(1+x+x^{2}+x^{3}+....+x^{27})(1+x+x^{2}+x^{3}+....x^{14})^{2}$ . Find the coefficient of $x^{28}$ I am not able to apply binomial theorem here
$(1+x+x^{2}+x^{3}+....+x^{27})(1+x+x^{2}+x^{3}+....x^{14})^{2}$ 
Please help me to find the coefficient of$ x^{28}$
Any help will be appreciated.

\begin{eqnarray*}
(1+x+x^{2}+x^{3}+\cdots+x^{27})(1+x+x^{2}+x^{3}+\cdots+x^{14})^{2} \\
= (1+x+x^{2}+\cdots+x^{27}) (1+2x+3x^{2}+\cdots +14x^{13}+15x^{14} +14x^{15} \cdots +2x^{27}+ x^{28}) \\
= \cdots+x^{28}( \underbrace{1+2 +\cdots + 15}_{120} + \underbrace{14+ \cdots +2}_{104})+ \cdots 
\end{eqnarray*}
EDIT

