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
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