-(IBAction)plusminusButton:(id)sender {  // Interface Builder action for plus (+)
    [self saveValueOfOperator:PLUSMINUS];
    [self saveValueOfArg1];
    [self calculateAnswer];

    [self setCalcAreaNumber:[NSString stringWithFormat:@"%f", calcAnswer]];  // float (double) to string
    [self setTextCalcAreaLabel];
    [self saveValueofAnswer];
}