check which operation to apply
if root . data == ' + ' : NEW_LINE INDENT return left_sum + right_sum NEW_LINE DEDENT elif root . data == ' - ' : NEW_LINE INDENT return left_sum - right_sum NEW_LINE DEDENT elif root . data == ' * ' : NEW_LINE INDENT return left_sum * right_sum NEW_LINE DEDENT else : NEW_LINE INDENT return left_sum / right_sum NEW_LINE DEDENT