Who carried the Aron?
Radak: It was the Leviyim, like Moshe commanded (Divrei ha'Yamim I, 15:15).
Why did they offer Korbanos after every six steps?
Radak: This is the amount that Uza went (before he went to catch the Aron, and died). They rejoiced that they did not stumble like he did.
Why does it say in Divrei ha'Yamim I, 15:26 "Hashem helped the Leviyim"?
Radak: They did not feel any weight on their shoulders. They knew that Hashem helps them.
What is "Meri"?
Radak citing Targum Yonasan: It is a fattened [calf].
Why does it say here that after six steps they offered a bull and Meri, and in Divrei ha'Yamim I, 26:15, it says seven bulls and seven rams?
Radak: Here it says "va'Yizbach" - David offered a bull and Meri. There it says va'Yizbechu - the Leviyim offered seven bulls and seven rams when they took the seventh step.
Radak citing Sotah 35b: After every step they offered an ox and Meri. After six steps they offered seven bulls and seven rams.
Radak citing Sotah 35b: After every six steps offered a bull and Meri. After six sets of steps they offered seven bulls and seven rams. Malbim - this is after 42 steps, corresponding to Hashem's 42-letter name. After six steps they offered a bull and Meri, to make the seventh Kodesh.
