2.64mg=___ng
31ul=0.031mL
5.1nl=0.0051ul
555nm=0.555um
2.5ug=0.0025mg
1ng=1x10^-9g

I did it but I am confusing on first one, Please check other answers for me...
I am confusing Thank you so much!

2640000ng , I think this is first one answer.correct?

Yes, all are good! :)