Answer
$a_1=-1$
$d=3$
Work Step by Step
With $a_3=5$, the value of $a_5$ can be found by adding the common difference $ d$ two times.
Since $a_5=11$, then
\begin{align*}
a_5&=a_3+2d\\\\
11&=5+2d\\\\
11-5&=2d\\\\
6&=2d\\\\
\frac{6}{2}&=\frac{2d}{2}\\\\
3&=d
\end{align*}
The value of $a_1$ can be found by subtracting $d$ from $a_3$ two times.
Thus,
\begin{align*}
a_1&=a_3-2d\\
a_1&=5-2(3)\\
a_1&=5-6\\
a_1&=-1
\end{align*}
