A surface ship is moving in a straight line horziontally at 20Km/hr. At the same time, a submarine is moving in the same direction at a 15 degree down angle staying directly under the ship. How fast is the submarines altitude decreasing.

$\displaystyle \frac{dx}{dt}=20$

$\displaystyle \theta=15$

What equation should I use? I tried Pythagorean identity and sin = y/r. Didn't yield anything promising.