Given at 11 °C = 95 mL and at 21 °C = 105 mL, then the rate of change of volume per degree rise in temperature is calculated as follows.
(V2 - V1)/(T2 - T1) = (105 mL - 95 mL) / (21°C - 11°C) = 10 mL / 10°C
= 1 mL / °C
Therefore, the average rate of change of f(x) between x = 11 to x = 21 is 1 mL/°C.