The range of a linear function can be found by evaluating the end-points, namely [-2,2] (assuming [-2,2] interval notation meaning from -2 to 2 was intended).
f(-2)=-6-12=-18
f(2)=6-12=-6
so the range of function f(x)=3x-12 for the domain [-2,2] is [-18,-6].
If the domain is actually {-2,2}, meaning only x=-2 and x=2, then the range would be {-18,-6}, i.e. exactly two points.