The number 121 is not a prime number. A prime number must have only two factors, 1 and the number itself. However, the number 121 has three factors as 11 is a also factor of 121 in addition to 1 and 121 itself.
A prime number is not divisible by any other number other than the number 1 and itself. To find out if a number is prime number its factors are obtained. The factors of 121 are 1, 11 and 121. Similarly, the factors of 43 are 1 and 43 only. This makes 43 a prime number. However, the number 50 has 1, 2, 5, 10 and 50 as its factors. Thus 50 is not a prime number but a composite number.