Calling values from a "dynamic" variable name

Calling values from a "dynamic" variable name

Hi,
sorry for the bad description, but I dont know how to name the problem, that's propably why I could'nt find a solution so far :(

The code looks like this:
  1. fi_slide = 1;
  2. TweenMax.to($('#fi_slide_'+fi_slide), animationLength, {css:{top:(sf_position_out+(["fi_slide_"+fi_slide+"_margin_top"])*(-1))+"px"},ease:Expo.easeOut});

So what im tying to achive is to set the css value "top" to a specific value. The variable I am tying to call has the name fi_slide_1_margin_top

But i am trying to call this variable dynamically, based on the previously defined variable " fi_slide = 1"
Obviously, my approach " ["fi_slide_"+fi_slide+"_margin_top"]" does not work. It just gives the string   fi_slide_1_margin_top as an output, but not the varibable value

Any solutions?