Since it's a right triangle you need to use the pythagorean theorem...

And since we are dealing with rates you need to derive the equation

The 2's cancel so we're left with

now what we're looking for is how fast the hypotenuse is changing,

and what we are given is

Use pythagorean theorem here to get

and

then we just solve

Hope this helps! it's my first time posting a reply so sorry if it's unclear, let me know if you have any questions