Baseball diamond related rates problem with player running from first to second

[nyr]

The problem is:
For the baseball diamond in exercise 33, suppose the player is running from first to second at a speed of 28 feet per second. Find the rate at which the distance from home plate is changing when the player is 30 feet from second base.
(the picture basically shows a square with 90 ft sides)

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My solution

Given: dy/dt = 28 ft/sec
Find: ds/dt when x=30

90²y²=s²
0 +2y(dy/dt)=2s(ds/dt)
2(60)(28)=2(30√13)(ds/dt)
56/(√13) ft/sec=ds/dt
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I'm not really sure if its the correct answer because I always seem to mix up negative signs and I wasnt sure if this required one.
If you really need to see the picture its found here:
http://books.google.com/books?id=E5...he player is 30 feet from second base&f=false
Page 151 #34

 

[Mark44]

The problem is:
For the baseball diamond in exercise 33, suppose the player is running from first to second at a speed of 28 feet per second. Find the rate at which the distance from home plate is changing when the player is 30 feet from second base.
(the picture basically shows a square with 90 ft sides)

__________________________________________________
My solution

Given: dy/dt = 28 ft/sec
Find: ds/dt when x=30

90²y²=s²

It looks like you left out the + sign on the left side. The equation should be
90² + y²=s²

0 +2y(dy/dt)=2s(ds/dt)
2(60)(28)=2(30√13)(ds/dt)
56/(√13) ft/sec=ds/dt

Looks fine to me.

__________________________

I'm not really sure if its the correct answer because I always seem to mix up negative signs and I wasnt sure if this required one.
If you really need to see the picture its found here:
http://books.google.com/books?id=E5...he player is 30 feet from second base&f=false
Page 151 #34