tax because the tax shows you what you paid and ect
The 6 terms of the series are
... 15, 14, 13, 12, 11, 10
The sum of them is 75.
_____
You could make use of the formula for the sum of the terms, or you could work out the sum from the given expression. It is easier just to list the terms and and them up.
There are 13 total hearts in the deck, which leaves 39 non-hearts. You're drawing a hand of five cards, so the probability of getting exactly 3 hearts is

as required.