Look for this textbook of material mechanics, which is usually in the appendix. For example, Liu Hongwen's "Mechanics of Materials" (no matter what version), Volume I, Appendix I, Section II generally has a detailed derivation process.

Netizen luoyisky007' s answer is a bit troublesome. Although this integral is the area integral of a rectangle, it can be calculated without double integral. The key is how to get to the micro-area dA. You can think of the micro-area as a slender rectangle parallel to the horizontal axis. The length of a rectangle is b and the width is dy. In this way, the integral becomes the integral of by3dy, and the value range of y is (-h/2, h/2). Just integrate it.