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You have a very small typo. == is for an equation, but = is for keywords. Try this.
var('x,y,z')
implicit_plot3d(x^2+y^2+z^2==4, (x, -3, 3), (y, -3,3), (z, -3,3)) + implicit_plot3d(x^2+y^2+1.75^2==4, (x, -3, 3), (y, -3, 3), (z, -3, 3))
