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## Homework Statement

The length l, width w, and height h of a box change with time. At a certain instant the dimensions are l = 7 m and w = h = 9 m, and l and w are increasing at a rate of 6 m/s while h is decreasing at a rate of 3 m/s. At that instant find the rates at which the following quantities are changing.

(a) The Volume

(b) The Surface Area

(c) The Length of the Diagonal

## Homework Equations

L

^{2}=l

^{2}+w

^{2}+h

^{2}

## The Attempt at a Solution

I have already done part a and part b but I'm having trouble with c.

I tried to work it out and got 42m/s but this is incorrect. Some help please?