Questions I Don’t get.

Hey guys new here. I’m from Nassau in the Bahamas. I’m a ninth grade student having trouble with math. In april I’ll be taking My Bahamas Junior certificates. Today I was tested to see how many hours of tutoring I need. I was given a past paper to complete. The lady said compared to a lot of other first timers I did pretty well. There were some questions I know I did in school but completely forgot they are the following please help me.

18.

A]= Use a pencil ruler and a compass to construct a circle with a radius of 4 cm. ( 1.6 inches, just about )

B}= Insert the following in the previous construction

Centre b

Diameter ABY

Radius BC

and the following question for the first one i am to find the value of r the second is how to find the value of g. could u guys sort of break it down for me >