word problem: A rectangular garden is to be surrounded by a

A rectangular garden is to be surrounded by a walkway of constant width. The garden's dimensions are 30 ft by 40 ft. The total area, garden plus walkway, is to be 1800 ft^2. What must be the width of the walkway to the nearest thousandth?

Let the width of the walkway be denoted by "w". Then the outer dimensions are given by:

(40 + 2w)*(30 + 2w)

Then the area equation is:

1200 + 140w + 4w^2 = 1800

4w^2 + 140w - 600 = 0

w^2 + 35w - 150 = 0

w = [-35+/-sqrt(35^2 + 600)]/2w = -35 + 24.49/2w = -5.25

My final answer doesn't match any. Do you know where I went wrong?
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Edited by stapel -- Reason for edit: formatting

w^2+35w-150=0
w=-35 +/- [35^2+600]^1/2 all over 2
w=-35+/-[1825[^1/2 all over 2
w=-35 +/- 42.72 all over 2
w=[-35+42.72]/2 w=[-35-42.72]/2 can't have negative walk
w=7.72/2
w=3.86

please check for errors
Arthur

again arthur thank you for your time and I hope you will be around next week when I need your help again. Thanks