When you flop a premium hand, you will have a difficult time extracting value, and when you flop a marginal hand, your opponent gets to decide how much money goes into the pot.
If you three-bet before the flop with A 7 and continuation bet this flop, your opponent will usually fold unless he improves to at least a pair, which will only happen about 35% of the time.
You will check the K-7-4 flop, your opponent will bet, and then you will fold. This will result in your opponent stealing pots from you.
I understand that playing aggressively may be uncomfortable at first, but you simply must learn to apply aggression if you want to succeed at poker, especially from out of position.
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