Algebra

First, let's set up variables to solve this question. 

Let's let $d = a_2 - a_1$

The conditions in the problem give us the system of equations

$\begin{cases} a_1 + d = -1\\ a_1 + 3d = \dfrac13 \end{cases}$

Now, solving this equation, we get that

$d = \dfrac23$ and $a_1 = -\dfrac53$

So the final answer is -5/3. 

Thanks! :)