increase a partition size

Using macOS 10.15. See attached screenshot. This is an external drive. The partition labeled "Seagate" is for time machine and I want to increase its size 100GB. The partition "External Drive" is just for normal files. The Untitled partition is from resizing the "external drive" partition. I can't combine that with the Seagate partition like I want. When I delete untitled it automatically combines with ExternalDrive.

How can I achieve this? Thanks