Help writing function

[Psyberion]

Hello everyone!

I need some help writing a function from a text. Here's the text:

A fisherman stands on a cliff 25m above the water. He's reeling in the fish with the velocity 1.5m/s. How fast does the fish move when the line is 40m long?

Could I have some help with this? I know that I have to calculate the derivate f'(40) for the function that describes where how far from the cliff the fish is, and I know that the function f(x)=sqrt(x²-25²). Thing is that X changes over time; if the line is C meter from the beginning, the line's length at t seconds is C-1,5t, but how do I calculate f'(40)=sqrt((C-1,5t)²-25²)?

I hope you understand what I mean, it's a bit tricky to explain, especially since English is not my native language. If I'm unclear, please tell me and I'll try to rephrase this.

Thanks a lot in advance!
~Psyberion

[AceInfinity]

Function for what? For graphing this relationship between the time and speed? I don't understand. The way I see it though it's a triangle, so it's basic Pythagorean Theorem ...

Answer would be 1.17m/s though.

[Psyberion]

Ok, I'll try to explain better.

So, you were right about the triangle. The line, represented in the picture by the hypotenuse, is reeled in 1.5m per second. So the length should be L-1.5t where L is a constant and t is the time in seconds, right?
The left cathetus is the hight of the cliff, so 25m at all times. The bottom cathetus is how far the fish is from the cliff. This length depends on how long the line is, thus it can be described as the function f(t) (sorry for the typo in the picture).

According to the Pythagorean theorem f(t)=sqrt((L-1.5t)²-25²), right? Or am I completely wrong about this? My teacher didn't agree with this, but it didn't sound like she was very sure though..

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I think I solved it, but thanks for the help!
~Psyberion