Problem with setting up Washer/Disk method for determining solid of revolution.

Need to find the volume of the solid of revolution:

Find the volume of the solid of revolution generated by revolving region about the line y=7

y=7sqrtx, y=7 and x=0

I set up the problem like this:

a=0 b=1 pi[7^2 - (7sqrtx)^2 dx] using the washer method

The answer to this problem is 49pi/6 in my book.

I got 49pi/2.

How am I setting up the problem wrong? These are difficult problems, and getting an answer does not help with the learning.

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