.

3 inches

Step-by-step explanation:

If we're gonna have some sort of increase each year, it's gonna pop up in our model attached to a variable, because it represents some kind of change, and variables can change. Looking at our equation, the 28.6 represents our starting point: the boy is 28.6 inches tall when he's 2 years old. The 3a term, then, represents the boy's increasing height each year. When he's 3 years old, a = 1, and so h = 3 + 28.6 = 31.6 inches. When he's 4, a = 2, and h = 3 + 31.6 = 34.6. The increase is always 3 inches.

b is the final answer! : )