Eight Digits Converter: Find the Missing Digits

This is a mathematical book and you must find the addition of two numbers with the given hint and the addition.
f.e. you have the question 1 : ( - 1 - ) + ( - 1 - ) = 828 (see first book Questions)
and you have to find the answer 1 : 216 + 612 = 828 (see second book Solutions).