You line up the two numbers on top of each other. This is called column addition. So you write the larger number on top and the smaller number on the bottom. Make sure that if the smaller number has less digits than the larger number, then it is aligned to the right so in the rightmost column there are always numbers present. So here we write 455 and 122 with 122 below 455. 1 should be below 4 and the 2's should be below the 5's in the three columns. Then we add the numbers in each column one by one, starting from the right. 5+2=7, 5+2=7, and 4+1=5, which gives us 577.
Level: Learning for fun