Okay, my prior answer got deleted for no work, apparently.
I'll re-post it though.
That word problem translates to:
C+a+b=p
move a to the other side
sign changes from +a to -a
C+a-a+b=p-a
C+b= p-a
move +b to the other side
C+b-b= p-a-b
Answer:
C= p-a-b