I have some questions in lab 1 exercise 8 the last question 

In my idea, the stack is built like follow
fmt     |  address
a        |   become 
b        |    small
c        \/

so, we can also use the fmt to find a, b, c, and just change all '+' in va_start and va_args  to '-'.
It's just my guess，and want to discuss with you, thanks.