'8.48528137 i' or 'unidentified'
-72 isn't actually possible because a positive number times a positive number is always a positive and a negative number times a negative number is always a positive number. So you could say "unidentified".
But in HiGh sChOoL you'll learn about imaginary numbers (according to my teachers), and how it works is; basically just find the square root of 72 and write 'i' after. 'I' stands for imaginary number.