Im trying to draw a shpere but it always saying: 

Error, (in plot3d) unexpected option: z = -2 .. 2

this is the equation: x^2+y^2+z^2-4=0

i'm writing this way

plot3d(x^2+y^2+z^2-2^2, x = -2 .. 2, y = -2 .. 2, z = -2 .. 2)

what should I do? this is my first time with this software

best from Brazil,
Nina