The highest point in the world is mt. everest at 8,850m above sea level. the lowest point in the world is the bentley trench in antarctica at 2,555m below. what is the difference between the highest and lowest elevation points?

Just subtract.
This is your formula.

8,850 - (-2,555) = ?  [It is negative 2,555 because it is BELOW sea level]
8,850 + 2,555 = ?     [The sign changed because a double negative is a positive]
= 11,405 m of a difference

77% of the school want electronic newsletters, so 23% of the school want physical paper newsletters.

77% of 650 is 500.5.
23% of 650 is 149.5.

650/100 = 6.5
6.5x77 = 500.5
6.5x23 = 149.5

It's best to round that figure up to 150 as there will not be a parent who only needs half of the newsletter.