Question: what is the square root of a negative 4?
Answer: TL;DR: It's undefined, or doesn't exist/is an imaginary number, because you can't take the square root of a negative number. A square is the result of a number, multiplied by itself; when the two are whole numbers, the result is a perfect square. For example, 16 is a square - 4*4 = 16 - and so is 25 - 5*5 = 25.