Actually, one can immediately see that 255 469 520 cannot be prime, because 5 is one of its divisors: indeed, a number ending with 0 or 5 has necessarily 5 among its divisors.
The last digit of 255 469 520 is 0, so it is divisible by 5 and is therefore not prime.
For 255 469 520 to be a prime number, it would have been required that 255 469 520 has only two divisors, i.e., itself and 1.
A number is a perfect square (or a square number) if its square root is an integer; that is to say, it is the product of an integer with itself. Here, the square root of 255 469 520 is about 15 983.414.