We want to calculate the sum of 645 and 763.
When we add 5 and 3 we get 8, so you write the 8 below.
Then you proceed to add 4 and 6 which is 10, so you will write the second digit 0 and keep the first digit 1.
then lasly add 6 and 7 which is 13 and then add the 1 we kept earlier then we have 14. so we can write the 14.
Therefore;