Hey there!
Your answer is 51.
First of all, a prime number is a number whose only factors are 1 and itself.
31 and 41 both fit the description of a prime number.
31's only factors are 1 and 31, and 41's only factors are 1 and 41.
However, in addition to 1 and 51, 51 has two other factors: 3 and 17.
This means that 51 is not a prime number.
Hope this helps!