The idea of representing the relationship between three variables of the form a = bc or a = b/c in a triangle is well established. Cover the variable you want as the subject of the equation and what is visable is the other side of the equation. Once you understand how the triangle works, you just need to remember the order of the letters. In the above example, the letters are written alphabetically, starting from the top: D, S, T.
Or alternatively, look at the first four letters in the spelling of the word DISTANCE. And think of the phrase Distance Is Speed by Time.