Related rates problem. Determine the speed of the plane given the following information. 
I started by noting that I am looking for $\displaystyle\frac{dx}{dt}$ and I am given $\displaystyle\frac{d\theta}{dt}=1.5^{\circ}/\mbox{s}$.
I then related $\theta$ to $x$ by $\displaystyle\cot(\theta)=\frac{x}{6000\mbox{m}}$, then taking their derivatives with respect to $t$, I get $\displaystyle\frac{dx}{dt}=-6000\mbox{m}\csc^2(\theta)\frac{d\theta}{dt}$, but i'm not sure if this is correct, this number seems too big, and it is also negative. Thoughts?

While your answer is still incorrect, the reason it is a negative number is because the value of $x$ is decreasing as the plane flies.
