45 is the starting charge, meaning that that is the default cost for any plumbing before they've started working. 35x means that it's 35 dollars per hour (x.) The 35 dollars per hour obviously depends on how many hours the person has worked. Y is the total cost of everything together.
The independent variable is x, since it depends on nothing.
The coefficient is 35, since it's the number before x.
The constant term is 45, since that number can't change.
The dependent variable is y, since it depends on x for the sum.
Hope this helps! :)