Use the indicated x-values on the graph of y = f(x) to find the following. Find intervals over which the graph is concave down. (Consider only the interval (a, h). Enter your answer using only interval notation.)

Suppose that a households monthly water bill (in dollars) is a linear function of the amount of water the household uses (in hundreds of cubic feet, hcf). when graphed, the function gives a line with slope of 1.65. if the monthly cost for 13 hcf is $46.10 what is the monthly cost for 19 hcf?