No, "your friend" is incorrect in that he/she subtracted 6 from 10 instead of multiplying 3 by 10. The bolded portion is incorrect 3m - 6 = 3 x 10 - 6 = 3 x 4 = 12. It should've read: 3m - 6 = 3 x 10 - 6 = 30 - 6 = 24
It’s - ( - 3 ) because a negative times a negative equals a positive so it wouldn’t be - ( 3 ) because it would turn into a negative number and - ( - 3 ) would be positive