I have Brother MFC 7840 W printer. Will I be able to print from iPhone X?

Brother MFC 7840W does not support AirPrint, which is the native printing for iPhone and iPad, and avoids needing vendor drivers.

Brother does list an add-on app that might work here.

https://support.brother.com/g/b/faqend.aspx?c=us&lang=en&prod=mfc7840w_all&ftype3=2047&faqid=faq00002727_000

For more on the Brother add-on software, see: http://www.brother.com/apps/iphone/index_pc.htm

this sounds like a question you should ask Brother.. Apple doesn't decide which printers it works with..

there are manuals here you could read thru

https://support.brother.com/g/b/manualtop.aspx?c=us&lang=en&prod=mfc7840w_all