Couple of things to lay the ground work.

Your statement is of the form

You want to find,

Apply the rules above.

Suppose , then a and b are odd.2) If the numbers a and b are primes, then the number a+b is composite.

For some then,

This implies,

Therefore a+b is even and not prime.

Now, if a or b is 2 then...this is not true. Consider the counter example with

then is prime (not composite)