How to make something invisible?

[yujiop]

so they name of the movie clip i want to make invisible is called menugroup_mc .
I want to make it invisible when you enter the frame, so i tried this code

menugroup_mc.addEventListener(EVENT.ENTER_FRAME, hideObject);

function hideObject (event:EVENT:(void {
menugroup_mc.visible = false;
}

but every time I check it it says "Scene 1, Layer 'Actions', Frame 1, Line 8 1084: Syntax error: expecting rightparen before colon."

line 8 is the "function hideObject (event:EVENTvoid {" line. Please help me fix it!
(im using flash cs6, actionscript 3)