Since the relationship it is linear we can find the equation that represents the growing of the plant, and then calculate the initial value. The linear equation form is: y= m*x + b. We need to calculate the slope (m) first, with a pair of points, let's choose (2,13) and (4,19).
Using the formula for the slope we have:
With the slope, which is equal to 3 and any point, let's use (2,13) we can find the parameter b.
y= m*x + b
13= 3*(2) + b (Replacing the values)
13 = 6 + b (Multiplying)
13 -6 = b (Subtracting 6 on both sides of the equation)
7= b
Since b is the y-intercept then it is the initial value, it means that 7 was the height of the plant when it was planted.