Moses and Aaron have done as God commanded, spoken to Pharaoh, yet the promise has not been fulfilled. Israel is still in bondage. To add "insult to injury" they are now tasked with delivering the same number of bricks, however not given straw as before, making their servanthood unaccomplishable. Often times, when God gives us a directive, it will get worse before it gets better.
The children of Israel have rebelled, and come against Moses and Aaron. Moses has rebelled and gone back to God in frustration and anger. Then God simply tells Moses to do just want He commissioned from the beginning; "Go in, speak unto Pharaoh king of Egypt, that he let the children of Israel go out of his land."
The very objection Moses had on the backside of the mountain, God recalls Him to the same task again. Why? Moses never overcame the challenge in his mind, of having enough faith in God, to know He could make it come to pass. If God commands us to do it, He will equip us to achieve it.
Notice also, it was not up to Moses to deliver the children of Israel out of Egypt, but simply to speak unto Pharaoh on God's behalf. God made this so easy for Moses, He even went as far to make Moses as a god unto Pharaoh. The Egyptians had a polytheistic society; they believed in many gods. They feared and referenced each one. This meant Pharaoh would hear Moses' words. He didn't actually need Aaron. But God wanted Moses to see and understand this for himself. He was trying to build Moses' faith in this area.
What thing did we not believe God for, and now He has brought you back to the same place. Are you now face with the objection you gave Him initially? Will you be overcome by faith or excuses? Can you believe it possible, that which you once could not see happening?
