Because you want to keep the partition that is lower in your partition list, there's no simple way to rejoin the space, as it exists now on your drive. If that backup partition is blank, then your can just erase the drive, which will utilize all of your space.
Or, if you have important files backed up on that backup partition, you will need to copy THAT data to another drive, then erase the big drive, and again, that will use all the space.