The word "sum" means addition, so the question is asking you to add 914 and 878.
First you can add the hundreds, so 900 + 800 = 1,700
Then you can add the tens, so 10 + 70 = 80
And finally the units, so 4 + 8 = 12
When you add up all of these, you get your final answer as 1,792.
I hope this helps!
