There are two digits in both numbers. Let's start with the ones digit.
The ones digit of 21 is 1 and that of 35 is 5.
We add 1 and 5.
We have the answer's ones digit.
There is no carry-over.
Next, we add the tens digits
Tens digit of 21 is 2, and that of 35 is 3.
We add 3 and 2.
Combining the tens digit and ones digit, we get the answer : 56.