Question 52531
The "Prime Factors" of a number are the factors of the number that are prime numbers. This sounds like a "circular" defintion so let's demonstrate with the numbers in your problem:

The prime factors of 38 are: 2 and 19.  Why?
Well, 2 times 19 = 38 so 2 and 19 are factors of 38 (and so is 1 of course but 1 is not considered to be a prime number)
Also, 2 and 19 are "prime numbers"
What is a prime number?
A number is a prime number if it has only two factors, 1 and the number itself.
Another way of putting it is:
A number is a prime number if it can only be divided (with no remainder) 
by 1 and the number itself.

Back to your problem:
The prime factors of 38 are 2 and 19
The prime factors of 84 are 2, 2, 3, and 7 (Check: 2*2*3*7 = 84)

x = 2+19 = 21
y = 2+2+3+7 = 14
x-y = 21-14 = 7