An example of a composite mathematical expression is 6x squared times 5x plus 15. To write this expression as a product of primes, we must factor it completely. Parts of this expression are trickier than others. 6 can be written as 2 times 3. X squared can be written as x times x. This expression, 5x plus 15, can be written as 5 times x plus 3. Now, putting our factors in order, we get 2 times 3 times 5 times x times x times x plus 3.