How to use an iMac 2011 as external monitor for my Macbook Pro 2018?

Thinking of using my idle iMac as an external monitor for my 2018 Macbook Pro. But not sure how that would work between the Thunderbolt 2/Mini Displayport on the iMac and the Thunderbolt 3 on my Macbook Pro.

Here are the ports on my iMac, as you can see the sign (second last from the right) there suggest it is a thunderbolt 2 port. What kind of cable do I need to use it as a monitor for my Macbook Pro?