Since we aren't specifying a noun here we use the feminine as a default. So literally "Would it not be better with you going now?".
I just typed "fasai hi ddim yn well gyda chi fynd nawr" and was marked wrong. Why? Is it because I didn't use a capital letter at the start, use a question mark or put a space at the end of the sentence?
Here the pronoun hi represents ‘it’. Literally ‘Would it not be better with you to go now’. However, this pattern in Welsh is used to convey the same meaning as the English ‘Wouldn’t you prefer to go now?’
This is explained in the course notes for the section. We recommend that people read the section notes before beginning each section - see this