(Part 1) the mean for algebra class is 41.
Just add all the numbers and divide by 7.
(Part 2a) the mean for art class is 82.
Since the mean grade for art is twice as mean grade for algebra, just multiply 41x2=82.
(Part 2b) the missing grade was 84.
I just did guess & check starting from 82 and that’s how I got my answer.