Answer:
The expression is 42-17-19 OR 42-(17+19).
Explanation:
First, we can indicate that we can use subtraction because is giving them away for his birthday.
"He gave one each to the 17 students in his class and to the 19 students in the other third-grade class".
That represents taking away 17 and 19 means that it is subtracting them.
Hope this helps. (: