the question is 408 - 390.
we have 8 - 0 = 8, so write 8 at the bottom of the units column.
0 is less than 9 so we add 10 to the 0 in the tens column giving us 10, and at the same time add 1 to be subtracted later from the hundreds column.
now we have 10 - 9 = 1, so write 1 at the bottom of the tens column.
we have 4 - 3 - 1 = 0, so write 0 at the bottom of the hundreds column. 