# Find the equation of the sphere

Find the equation of the sphere centered at (-6,-7,-1) with radius 9.

well i got $$(x+6)^2 + (y+7)^2 + (z+1)^2-9^2$$ which is correct.

now for the next question:

Give an equation which describes the intersection of this sphere with the plane $$z=0$$.

i dont understand how to do the second question at all, can someone help?

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What you got must've been:
$$(x+6)^2 + (y+7)^2 + (z+1)^2-9^2 = 0$$.

The equation that describes the intersection with the plane z = 0 must be:
$$(x+6)^2 + (y+7)^2 + (0+1)^2-9^2 = (x+6)^2 + (y+7)^2 - 80 = 0$$

thanks alot, i get it now