Calculating rate of change of momentum for a package

[EmSeeSquared]

A helicopter is flying to the right (in the +x direction) with a constant speed v, carrying a package of mass M suspended below it using a cable as shown in the figure (you can assume that H and L are not changing with time).

Choose the package as your system. What is its rate of change of momentum?

So, what I figured out is to calculate the angle between the string and the package using H and L. I draw a triangle and find the angle using tanx = H/L, then x = arctan(H/L)

After I get the angle I can resolve to find the resultant force. But how do I get the rate of change of momentum with that?

 

[Simon Bridge]

What speed is the package doing?
Is the package accelerating?