Because it is a prime number, it can be expressed as 6k+ 1 or 6k- 1.
(6k+ 1)^2=36k^2+ 12k+ 1= 12k(3k+ 1)+ 1
So whether k is odd or even, k and 3k+ 1 must be odd and even.
So the square of 6k+ 1 minus 1 is a multiple of 24.
Similarly, (6k-1) 2 = 36k2-12k+1=12k (3k-1)+1.
K and 3k- 1 must be odd and even.
So the square of 6k- 1 minus 1 is a multiple of 24.
So the square of prime number-1 must be a multiple of 24.