Create a function that outputs the results of a flashcard. A flashcard is a list of three elements: a number, an operator symbol, and another number. Return the mathematical result of that expression.
There are 4 operators: + (addition), - (subtraction), x (multiplication), and / (division). If the flashcard displays a number being divided by zero, e.g. [3, "/", 0], then return None. For division, round to the hundredths place. So [10, "/", 3] should return 3.33.
flash([3, "x", 7]) ➞ 21
flash([5, "+", 7]) ➞ 12
flash([10, "-", 9]) ➞ 1
flash([10, "/", 0]) ➞ None
flash([10, "/", 3]) ➞ 3.33
Flash cards contain only zero or positive numbers.