To find the weight of the puppy when it is 15 weeks old, we need to substitute 15 for x in the equation of the line of best fit:
y = 2x + 1.5
y = 2(15) + 1.5
y = 31.5
Therefore, the weight of the puppy when it is 15 weeks old is 31.5 units, where the units depend on the units used to measure weight (e.g. pounds, kilograms, etc.).