this crack is quite long, may not be worth fixing.

https://www.google.com/search?sca_esv=e9f3975971afe82a&sca_upv=1&rlz=1C1GCEU_enUS901US901&q=how+to+repair+cracked+fiberglass+tub&tbm=vid&source=lnms&prmd=visnbmtz&sa=X&ved=2ahUKEwjeyb6wm4KFAxVCkokEHR8MA1gQ0pQJegQIDhAB&biw=989&bih=635&dpr=1.75#fpstate=ive&vld=cid:6fd7acaa,vid:6Q8T2LJleUc,st:0

if you want, try this, with a thick coating of patch, to cover at least 1 inch from the crack in all directions. I would also 

put a metal mesh into the compound to increase the strength to prevent the cracking from coming back. The compound itself does not have enough strength.

you also have to spray a glaze of matching color after that.