Basically the high temperature,T, is a function of the day of the month, D. You know this because, T is not an independent variable and is determined by the day, D.Aka, you need to use D in order to figure out T. Remember that any variable dealing with a specified amount of time is equivalent to your x value and the dependent variable is equivalent to your y value.

So you would input the day of the month, D, or you can also think of it as x, to get the high temperature, T, or y.

Since you know now that T is a function of D, or f(D) = T [ f(x) = y ], you can say that on the 12th day of the month the high temperature was 78 degrees F.