Ask a few people you know if they have heard of the equation. If they have, it probably falls into "common use" and you can write the equation without attribution. Many equations fall into this realm, including common trigonometric and geometric equations like the Pythagorean theorem and equations for the areas of geometric shapes.
Find out how old the equation is. If the equation has been in use for centuries, there's no need to cite a source. A more recent equation (within the last century), probably needs attribution.
Cite the equation's original source in your paper by including a parenthetical citation (a parenthetical citation is information that comes after the equation, with the name of the author and year published).
Include a reference page or bibliography at the end of your paper. That page should list enough information about the reference so that another party could locate the item. The Owl at Purdue states that the following information is needed to properly cite a book in APA format (APA is commonly used for mathematics):
Author, A. A. (Year of publication). Title of work: Capital letter also for subtitle. Location: Publisher.