The number 39 is not a prime number. The definition of a prime number states that it is a number that only has one and itself as factors. Because 39's factors are one, three, 13 and 39, it does not conform to the definition of a prime number.
The number 39 is a composite number because it has more than two factors. To find the factors of 39, a person has to find divisors that divide evenly into 39. One method used to find these factors is through divisibility tests. The procedure is to begin with two and see if it divides evenly into 39. Since two does not divide evenly into 39, the next numbers to try are three, four, five, six, seven, eight, nine and 10. In this manner, it is simple to find all the factors of 39.