Hello. The answer is p = 3 + 1/3x.
Basic slope-intercept form is y (in this scenario, p) = mx + b. M represents the slope and b is the y-intercept. They want you to write it as y = b + mx. The vertical intercept, also known as the y-intercept, gets placed first as its the starting height. The height starts at 3 inches tall. The slope would be the growth in inches per month, 1/3. Hope this helped!