I need help with a and b

Whats the name of the vertical axis

In a graph with several intervals o data how does a constant interval appear? what type of scenario produces a constant interval?

Dean and karlene get paid to write songs. dean is paid a one-time fee of $20 plus $80 per day. karlene is paid a one-time fee of $25 plus $100 per day. write an expression to represent how much a producer will pay to hire both to work the same number of days on a song.