Compute the accurate solution of the following question

Question- Consider a planetary nebula at a distance d. We measure with the Doppler effect that the shell is expanding at a speed of 25 km/s and find from decade-long observation that the diameter of the nebula increases by 0.75 arcsec per century.

Part 1- Calculate the distance to the nebula in light-years.

I need assists to find the distance to the nebula in light-years.