An expanding shock wave generated by a supernova is observed as a circular?

shell structure located at 400 pc from Earth, and the shell has a diameter of 31.4 pc. (i) Find the current angular diameter (deg) of the shell on the sky. (ii) The shell is expanding with the speed of 1,570 km s-1. How many years ago was the supernova explosion possibly observed on Earth? (iii) How long would an astronomer on Earth have to wait to see the radius of the shell expand outward by 1 arcmin? [ume that the radius of the shell expand at the same constant speed. Also use 1 yr = 3x107 sec.]