Hi @donnac1,
A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.
I can see that you have removed the handle and collar from a mixer tap, exposing the mixer cartridge.
Can you advise what you are struggling with at this point?
Is it removing the mixer cartridge?
If so, then you can use a shifter or multigrip pliers to grip the flat section of the nut shown below.
Once gripped, rotate counterclockwise to remove the cartridge.
You can then take it in store with you to match up with a suitable replacement, reversing the steps taken to this point to reinstall the cartridge.
If this is not what you are struggling with, please let me know so I can assist further.
Jacob
