lol wow, crazy parent.
okay, think about what you wanna do after highschool, whether you wanna go tafe and do a trade, or get a cert. in business or something, OR go to university to get a maths/science/commerce/engineering/law/medical/whatever degree.
THEN think about whether 3U maths is required in whatever you wanna do after highschool.
if you wanna do a degree that has 3U as a requirement, then study up, get your teachers to help, and maybe get tutoring.
if you wanna do commerce or something that doesnt have 3U as a requirement, then imo there is no point in taking an additional unit that won't count towards your future tertiary education, and you should let your dad understand that instead of spending time on 3u, you're better off spend time studying more for your other subjects.