Confused about my LTCG for house sale

I purchased my first flat on May 2015, my second flat on Mar 2021.

Planning to sell first one by  Oct 2022.

What's the tax implications and how to offset against the second flat purchase.

While i am clear on first sell and then purchase laws but not getting any satisfactory answer for sale after purchase of a new property.

Kindly assist with correct rules here.