[fmx pro] Left side of assignment operator must be variable error problem

I’m somewhat new and trying to learn actionscript. I’m getting and error that has me puzzled.

Error Scene=Scene 1, layer=actions, frame=1:Line 2: Left side of assignment operator must be variable or property.
bg-mb_mc._alpha=50;

I have a timeline with 3 jpgs on it that have been converted into symbols.

bg-gooto_mc is an instance of bg-gooto
bg-mb_mc is an instance of bg-mb
bg-lace_mc is an instance of bg-lace

When I try … bg-mb_mc._alpha=50; I get the error.
using setproperty ("_level0.bg-mb_mc", _alpha, 45); works ok.

It would be greatly appreciated if someone would please explain the problem.