If I'm correct, you're asking which variable is the dependent variable y from the question
Answer and explanation:
The equation can be modelled:
y= 1.5x
Where y is the current length of the hair
x is the number of months of hair growth
We can see from the equation therefore that y is a dependent variable that depends on x the independent variable, the number of months, to get a value. In other words given that her hair grows at a constant rate of 1.5 inches per month, her hair length is based on number of months x of growth in inches of hair.