Need help with listbox texts to have a value and add onto another textbox

Basically, let's say in listbox1 I put a code staying listbox1.items.add(" lmao =$50")
Listbox1.items.add("xD = $30")

So, what I want is that when running this build when I select both of these on the list box I want a textbox to have the value 80 (50+30)

I can voice call on discord if required but I really need help to finish my assignment, I'm freaking out.