Answer
$a_1=\dfrac{2y}{n}-x$
Work Step by Step
Let $a_n$ the arithmetic sequence representing the number of seats in each row.
We are given:
$$\begin{align*}
a_n&=x\\
S_n&=y.
\end{align*}$$
Rewrite the above equations using $x$, $y$, $d$, $n$:
$$\begin{align*}
a_n&=x=a_1+(n-1)d\\
S_n&=y=\dfrac{n}{2}(a_1+a_n)=\dfrac{n}{2}(a_1+x).
\end{align*}$$
Solve the second equation for $a_1$:
$$\begin{align*}
y&=\dfrac{n}{2}(a_1+x)\\
2y&=n(a_1+x)\\
\dfrac{2y}{n}&=a_1+x\\
a_1&=\dfrac{2y}{n}-x.
\end{align*}$$
