Answer
See below.
Work Step by Step
We then know that $x-(3-x)=1-3x-x\\2x-3=1-4x\\6x=4\\x=\frac{2}{3}$
Then the first three terms are: $\frac{7}{3},\frac{2}{3},-1=\frac{-3}{3}$, thus we can see that the common difference is $\frac{-5}{3}$. Thus the next term is: $\frac{-8}{3}$
