The principle you're supposed to extract from D&C 57:7-13 is
We should use our individual strengths to help build the Lord’s kingdom as He calls upon us to do so.
(Not the only principle, probably. But it's the one the
manual suggests.)
Then we ponder: what are my strengths?
When discussing this with seminary students, it can be helpful to remind them: you are yet early in the process of fully realizing your strengths. In the next few years your full complement of strengths will become apparent to you.
(Although, since 'full complement' seems like it could be a little heavy for high schoolers, you might word it differently.)
But what about me? I'm a little bit past the "next few years" that I was supposed to discover my strengths. Am I stuck with what I've got?
I read a blog post by a friend (acquaintance? How much do you have to hang out with someone before they count as a friend?) about how she has realized that she has more to learn, but that where she is now is where she needs to be. She may have strengths she'll yet discover. But where she needs to be, she has the tools she needs.
I still remember a stake conference where one of the old men (can't remember his calling exactly) saying that in his patriarchal blessing he was blessed with an affinity for family history. Finally he was like 60 and realized that this strength hadn't come to him yet. So he started. And it was awesome.
So there's hope for all of us.