My question is similar to Avik's but with different values.

A camera, located 4 km from the launch pad, is tracking the rocket that is fired straight up. When the height of the rocket is 13 km, the camera is rotating at the rate of 3/169 radians per second. What is the speed of the rocket at that instant? Give your answer in km/sec

I do not know how to figure out the value for sec^2 theta.
Thanks for all the help.

answered above.