Question 19
In the expansion of $$
\begin{aligned} &\left(1 + x + x^2 + \cdots + x^{27}\right) \\ &\quad {}\cdot \left(1 + x + x^2 + \cdots + x^{14}\right)^2, \end{aligned}
$$ what is the coefficient of $x^{28}?$
\begin{aligned} &\left(1 + x + x^2 + \cdots + x^{27}\right) \\ &\quad {}\cdot \left(1 + x + x^2 + \cdots + x^{14}\right)^2, \end{aligned}
$$ what is the coefficient of $x^{28}?$