\begin{equation*}
\begin{split}
y & = \frac{1}{\theta + 3 \theta ^2} \\\\
y & = (\theta + 3 \theta ^2)^{-1} \\\\
\frac {dy}{d\theta} & = (1 + 6 \theta)^{-2} \\\\
\frac {dy}{d\theta} & = (1 + 6 \theta) \:.\: (\theta + 3 \theta ^2)^{-2} \\\\
\frac {dy}{d\theta} & = \frac {1 + 6 \theta}{(\theta + 3 \theta ^2)^{2}}
\end{split}
\end{equation*}
\begin{equation*}
\begin{split}
x & = (1 - \theta)^2 \\\\
\frac {dx}{d\theta} & = -1 \:.\: 2 \:.\: (1 - \theta) \\\\
\frac {dx}{d\theta} & = -2 + 2 \theta
\end{split}
\end{equation*}
\begin{equation*}
\begin{split}
\frac {dy}{dx} & = \frac {\dfrac {dy}{dt}}{\dfrac {dx}{dt}} \\\\
\frac {dy}{dx} & = \frac {\dfrac {1 + 6 \theta}{(\theta + 3 \theta ^2)^{2}} }{-2 + 2 \theta} \\\\
\frac {dy}{dx} & = \bbox[5px, border: 2px solid magenta] {\frac {1 + 6 \theta}{(\theta + 3 \theta^2)^2 \:.\: (-2 + 2 \theta)}}
\end{split}
\end{equation*}
