Different people find different things hard. So a problem that is hard for someone may seem easy to you and one that you think is hard may be easy for someone else. It is, therefore, not possible to answer the question.
The answer to an addition problem is the sum of the problem.
I am pretty sure you can make them as hard as you want - if you have a "hard" problem, you can always find one that is even harder. If this is true, then there is no such thing as a "hardest algebra problem".
it is the sum.
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The answer to an addition problem is called the sum.