The sum of two even numbers will always be even. The sum of two numbers refers to the result of adding them together.
An even number is defined as any number that has 2 as a factor. For example, 2, 4, 6, 8 and 10 are all even numbers. Any number without 2 as a factor is odd, like 3, 5, 7 and 9. Because 2 is a factor of all even numbers, it can be factored out. The sum of x+y then looks like this: 2*(x/2+y/2). No matter what the numbers inside the brackets are, after being multiplied by 2, the final result will then have 2 as a factor, which makes it even.