Asymmetric bending principle of second moment of A derivation

[yonese]

TL;DR

I don't understand how the radius and offset of the Mohr's circle can be simplified down to the principal Iq and Ip equations - where does the load F go? This was taken from a worked example with inverted L section carrying a load. My teacher hasn't explained how he jumps to these steps and I don't understand how the load F gets removed and the 6 simplifies to a 3. I have tried to solve for simultaneous equations using the radius and offset equation, but still stuck. Thanks.

 

[Dr.D]

Perhaps you could post a picture to show what problem you are considering?