Having trouble with #3A-D on page 4, not understanding how to use 78.75 pixels to get “a” true diameter of ring in parsecs, “b” radius of ring in parsecs, “c” diameter in km, “d’ radius in km.

also question #7 A-F on page 4, dont understand how i am using 5 years and 3 month to give me my answers for speed of shockwave in km, distance of ring in km/s, “b” is just completely confusing to me, second in year s/yr,”e” and “f” i just dont know how to answer them