Final answer:
The rate of change in the baby's weight from three to six months is 2 pounds per month.
Step-by-step explanation:
The student has asked about the rate of change of a baby's weight between three and six months. To calculate the rate of change, which is the weight gain per month, we subtract the initial weight from the final weight and divide by the number of months. The baby weighed 12 pounds at three months and 18 pounds at six months. So, the baby gained 18 pounds - 12 pounds = 6 pounds over the course of 6 months - 3 months = 3 months.
The rate of weight gain is therefore 6 pounds / 3 months = 2 pounds per month.