This is a basic exercise in assigning oxidation numbers to simple compounds and ions. When you press "New Problem", a compound will be displayed and you will be asked to assign an oxidation number to one of the atoms present. Enter the value in the cell and press "Check Answer". If the oxidation number is positive or 0, just enter the value; but if the oxidation number is negative, be certain to include the sign and put it before the number, i.e. -1 not "1-". Pressing "Show Answer" will display the correct value and you will no longer be able to enter an answer for that question.