26 is not a prime number because?
For 26 to be a prime number, it would have been required that 26 has only two divisors, i.e., itself and 1. However, 26 is a semiprime (also called biprime or 2-almost-prime), because it is the product of a two non-necessarily distinct prime numbers. Indeed, 26 = 2 x 13, where 2 and 13 are both prime numbers.