HDD Partition resizing question

hi all,
i've read few articles how partition , resize hdd partitions haven't understood couple of things.
have external 2tb drive partitioned in 3 ways: 1tb use tm backups, 900gb used various staff , 100gb exfat partition, in order they're written. i'd delete third 1 , reduce @ bare minimum if not delete second partition, 900gb one. know deleting simple disk utility, problem comes resizing it, example, 900gb 300gb, , therefore giving more space first partition backups.
i've read articles said i'd have input new "size" in disk utility, others said i'd have delete partition , re-create it... i'm quite confused.
can me here?

 

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