Finding equilibrium concentrations, trouble simplifying equation
Everything you do is correct and it is only the final algebraic transformation missing:
\begin{align}
0.068 &amp;= \frac{1-x}{(1.25+x)(2.00+x)}\\
0.068 &amp;= \frac{1-x}{x^2 + 3.25x + 2.5} &amp;&amp;|\cdot (x^2 + 3.25x + 2.5)\\
1-x   &amp;= 0.068(x^2 + 3.25x + 2.5) &amp;&amp;|-1 +x\\
0.0   &amp;= 0.068x^2 + 1.221x -0.83\\
\end{align}

Now you can solve for $x$ according to
\begin{align}
0&amp;=ax^2+bx+c &amp; x_{1,2}&amp;=\frac{-b\pm\sqrt{b^2 - 4ac}}{2a}\
\end{align}

And that gives me the values $x_1\approx0.656$ and $x_2\approx-18.6$. The latter one of course makes no sense.