Combine place value strategy is useful if you try to subtract a number with bigger digits from a number with a smaller digit in the same place... such as now, where 3 is smaller than 9 in the ones place.
Then we can combine two place values: 23 and 19 and subtruct those. 23-19 is 4 (you can just count up from 19 to 23), so you know that the two will give the result of 04, and then you can proceed to the left: 2-1=1.
