Series is 19, 24, 21, 23, 28, 24 (from the question title).
First differences:
+5, -3, +2, +5, -4. Since +5-3=+2, perhaps the next three differences would be +5-4=+1 making the next number 24+1=25.
Or, if the series was supposed to be 19, 24, 21, 23, 28, 25 (from the question details) with first differences:
+5, -3, +2, +5, -3, the next difference would also be +2, to match the first pattern, so the next number would be 25+2=27.