The sum of two numbers is what you get when you add them together.
8+492 is the same as 492+8. 8+2=10. That's 1 to carry over into the tens column.
But there's a 9 in the tens column and 9+1=10, so 1 has to be carried over into the hundreds, where there's 4. 4+1=5.
So we have 5 in the hundreds and zeroes in the tens and units: 500.