1)
What will Yir'ei Hashem see in him?
1.
Rashi: They will see the good that I have.
2.
Radak: I understand everything.
3.
Malbim: A sign from Hashem for the good.
2)
Why will they rejoice?
1.
Rashi: Like my reward will be theirs, for I am among those who fear You, and I hope to Your word.
2.
Radak: You gave what I hoped for - Your word and help.
3.
Malbim: Due to the good sign they see in David. Until now, he requested it lest his revilers and haters be haughty over him. Now he requests for another reason - so Yir'ei Hashem will rejoice. If they see that evil befalls me, they will be sad.