Command to release the best amount of Kernel Task and Memory Ram

There's no ordinary system command for that purpose - simply because it is not what you would normally want.

The memory used here will help the performance of your system by caching data that might be needed later. If it is not needed later, because you need to load something different in RAM, the caches will be released automatically.

However, you can simulate such a command by simply making a small program that allocates progressively larger amounts of memory until you have cleared up all caches. There's no real point in doing so however as it won't help your system run faster, or be able to run more programs or anything like that.