<span>Northwest is 45 degrees north of west. If a car travels a distance x miles northwest, its northerly component gains x/sqrt(2) miles and its easterly component LOSES xsqrt(2)/ miles.
Resultant height is h = 4500 + 100x/sqrt(2) - 75x/sqrt(2) = [25/sqrt(2)]x + 4500
dh/dt = [25/sqrt(2)]dx/dt = [25/sqrt(2)]v </span>

First we must apply the Quotient rule that states,

This means that our derivative becomes,

Now we need to calculate
and 


From here the new equation looks like,

And that is the final result.
Hope this helps.
r3t40