2,2,5, 13,28,?

The pattern is + 0, + 3, + 8, + 15, ..... i.e. + (l² - 1), + (2² - 1), + (3² - 1), + (4² - 1), .....

So, missing term = 28 + (5² - 1) = 28 + 24 = 52.