Hello everyone having some troubles with maths.

I am a stationary object 150m below the waters surface.

I observe an object that is moving towards the waters surface directly above me.

I observe that the distance between me and the object is decreasing at 5m/s when it is 250m away from me.

The objects speed is constant.

Find the objects speed.

I cannot figure out how to solve this problem. I have looked around but always get stuck. If possible I would really appreciate it if someone can show me how to solve problems of this type. You don't have to give me answers to my question specifically. I'm more concerned with the method.