webwork question

 Find the center and radius of the sphere
x2+20x+y2+8y+z2−20z=−180
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How do you find the radius? I completed the square for x, y, and z, but I
couldn't find the radius.

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***** Data about the problem: *****

Problem ID:                   9
Source file:                  Local/Rochester/setVectors1space3D/UR_VC_1_5.pg
Value:                        1
Max attempts                  unlimited
Random seed:                  1623
Status:                       0.75
Attempted:                    yes
Last answer:
        AnSwEr0001: 4
        AnSwEr0002: 8
        AnSwEr0003: -2
        AnSwEr0004: sqrt(17)
Number of correct attempts:   0
Number of incorrect attempts: 11
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When you've completed the square you should have something like

(x-a)^2 + (y-b)^2 + (z-c)^2=S

for some numbers a,b,c and a number S≥0.  The numbers (a,b,c) are the coordinates of the center of the sphere and S is the square of the radius, i.e. r=√S.