Placing equation numbers can be a rather complex problem in multiline displays. The environments of the amsmath package try hard to avoid overprinting an equation number on the equation contents, if necessary moving the number down or up to a separate line. Even so, difficulties in accurately calculating the profile of an equation can occasionally result in a number placement that doesn't look right. So there is a raisetag command provided to adjust the vertical position of the current equation number. To move a particular number up by six points, write |6pt|. (This kind of adjustment is fine tuning like line breaks and page breaks, and should therefore be left undone until your document is nearly finalized, or you may end up redoing the fine tuning several times to keep up with changing document contents.)