Adding an operator "-" dilema

Been trying to figure out how to modify these line 4&5:

        // x postion mainBtn
        mainBtn._x += mainMenu["mainBtn"+(i-1)]._x+mainMenu["mainBtn"+(i-1)].btnName.t.textWidth+mainSpacing;
        // scale over and normal backgrounds to fit length of text = mainBtn.btnName.t.textWidth+mainSpacing;
        mainBtn.overMc._width = mainBtn.btnName.t.textWidth+mainSpacing;
        // center text in middle of button
        mainBtn.btnName._x =;

Been trying to write/add on a rule subtracting pixels from the width size of the button movie clip ( Hence which is automatically create/made up from the length of the width size of the button. The buttons are dynamic from XMl doc. Purpose is I’m trying to shave down a 20 pixels of the automated width.

Can anyone help?