The independent variable is a constant that does not rely on anything. This will be $5 because she has to pay it every month regardless.
The dependent variable change and affect the outcome. This will be $1.50. She has to pay per movie. If she doesn't watch a movie she doesn't pay anything on top of the $5. If she watches 10 movies, she payed $15 on top of the $5.
The slope of the given line is -3. Since perpendicular lines have slopes that are negative reciprocals of each other, the slope of the line we need to find is 1/3.