Why does he mention that Hashem is "Rachum v'Chanun"?
Radak: You will have mercy on me and benefit me.
Malbim: This is connected to the next verse. Because you are "Rachum v'Chanun", turn to me, to see my affliction, and Chaneni.
Why does he mention that Hashem is "Erech Apayim"?
Radak: If I sinned, You will delay Your anger against me, and not punish me until I fully repent, and You will pardon me, for this is Your Midah.
Why does he mention that Hashem is "Rav Chesed ve'Emes"?
Radak: You will increase Your Chesed and Emes to me. Emes with David is to fulfill His word about the kingship.
Malbim: This is connected to the next verse. Due to Your Emes, give Your Oz (strength) to Your servant. Due to Your Chesed, save the son of Your Shifchah. I.e. he serves Hashem (a) due to his choice - therefore it says "Avdecha"; (b) he received this nature from his fathers - therefore it says "l'Ven Amasecha."
