Bug fix 2
Basic Reset Button

Split the card generation function into two; one generates an ArrayList
of six values, and the other assigns them to the cards. The reset button
was added to regenerate the cards...
Reset Button additions

The reset button will now undo all changes in addition to resetting the
values.
answer can now equal -1
Merge branch 'resetButton' of github.com:himat/Math-Game into resetButton
fixed calculation bug with -1
Reset Button restores cards/operations

Can someone please check if this code works (does not cause bugs..)?
Undo Bug Fix

Undo button previously restored every instance of a repeated card (ex.
if there are two 3's, it will restore both 3s instead of one).  This
issue has been fixed; Each card generated from home now has a "tag" on
it to show which slot it belongs to.