its grand i sorted it out myself.i was actually right and foud out that if you change

}
function calculateAnswer(event:MouseEvent) {
if (theOperation == "+") {
theResult = Number(theResult) + Number(enteredNumber);
txt_result.text = theResult;
//*you just add another if statement//*
if (theOperation == "-") {
theResult = Number(theResult) - Number(enteredNumber);
txt_result.text = theResult;
}
//* it was really that simple//*