Completing the square x^2-19x-42=0

What is the missing step in this proof ?

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.

Assume that y varies inversely with x. if y=1.6 when x=0.5 find x when y=3.2