Undefined Derivative of a Parametric Function

A curve is defined by the parametric equations $x(t)=\ln\Bigl(\frac{t+3}{t+1}\Bigr)$ and $y(t)=\sqrt{\frac{t^2+2t+1}{t^2+6t+9}}$. For which of the following values of $t$ is the derivative $\frac{dy}{dx}$ undefined?