At noon, ship a is 170 km west of ship b. ship a is sailing east at 35 km/h and ship b is sailing north at 20 km/h. how fast is the distance between the ships changing at 4: 00 pm?

Check the picture below.

now, keep in mind that ship B is going at 20kph, thus from noon to 4pm, is 4 hours, so it has travelled by then 20 * 4 or 80 kilometers, thus b = 80.

whilst the ship B is moving north, the distance "a" is not really changing, and thus is a constant, that matters because the derivative of a constant is 0.