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my rear delts r weak and my bent over barbell row r also weak what r some good exercises for them to get strong or will it just take time

It does take time. You have to do the bent over row right too. You don't want to be bent at 90 degrees or almost standing. It is kind of in between, the bar should come just a bit above the waist too. This is to get the whole back evenly. Even with that it doesn't quite get the lats hard enough. Add a set or 2 of pulldowns or pullovers. Do a set of rear delt rows too. Use a wide grip, bend at 90 degrees with bent knees. Then lift it straight up and down from the floor to your shoulder/collar area. If your goal is strength, heavy weight and low reps just short of failure is the way to go.