The question asks "The kinematic equation d=vt is used in conjunction with relative velocity questions. Explain why the other kinematic equations are not useful for most relative velocity questions." I assume the answer to this is that most relative velocity questions that I deal with assume that acceleration is 0 (constant velocity) and therefore d=vt is the most useful of them. The answer that I was provided with at the back of the booklet was simply, "a =/= 0". Thanks for your help!