a force of 10 lb is required to hold a spring stretched 4 inches beyond its natural length. How much work is done in stretching it from its natural length to 6 inches beyond its natural length?

ok, i was just able to answer my own question and get the right answer, which is 15/4 ft-lb.... originally i got 25 in-lb .... is that equivalent? and is that a valid unit of measurement. I ask because I havent taken physics, and im curious as to why the book automatically converts the answer to feet when the question is in inches... also i believe the professor mentioned something about the bounds should avoid 0- does that make any sense or do you know what he was talking about?

any help appreciated- basically im just wondering if you can use inches