the board. in other words, pentagon b - pentagon a = pentagon c.
for example, take the top row of numbers. a = 2, b = 5, and c = 3. plugging in those numbers to our formula (b - a = c), we get 5 - 2 = 3, which is correct.
now let's use it to solve for the missing number in pentagon b, the bottom right.
see also: this brain teaser has a missing number — can you find it? 