By itself, the question makes no sense. It's like adding an exponent to the addition symbol. The "dx" is just a symbol, stating what variable you're integrating by. I'm not sure if there is much of a general "second integral" formula or not, but it's not denoted as (dx)^2. I think if you want to express a second order integral, the best thing to do is to express the equation as a differential equation (i.e., if y is the 2nd integral of f(x), then write it as y'' = f(x))