To solve for y we need to use d INVERSE OPERATION of addition which is subtraction. Then we will check our work:-
Lets do it:-
78 + y = 523
523 - 78 = 445
y = 445
CHECK OUR WORK:-
78 + y = 523
y = 445
78 + 445 = 523
We were RIGHT!!
So, y = 445
Hope I helped ya!! xD