Let us take some examples:

Ex 1:    4729  -  _936 = 2793

Solu:

Start on the right. Subtract the ones, tens, and hundreds columns, remembering to regroup.

Look at the thousands column. You want to find a number that, when subtracted from 3, gives a difference of 2

i.e. 3 – 1 = 2

So, the missing number is 1

Hence, 4729  -  1936 = 2793

Ex 2: 739  -  _09 = 630

Solu:

Start on the right. Subtract the ones and tens columns.

Look at the hundreds column. You want to find a number that, when subtracted from 7, gives a difference of 6

i.e. 7 – 1 = 6

So, the missing number is 1

Hence, 739  -  109 = 630