I've never been very good at math and confuse very easily so I need some help in doing a calculation regarding a circle. 

Let's assume we're drawing the letter L. 

The top point $A$ of the long leg is at $0$ the bottom point of the long leg $B$ is at $3$ feet and the point to the right $C$ is at $6$ inches. 

What would the diameter of a circle be if it was drawn so that $A$ and $C$ were connected? Can you tell me in lay terms how to make that calculation or some semblance of usable formula that I might comprehend, please?