Answer:
k = 0.381cm/gallon
Explanation:
Let the amount a tree grows in one month be x
Let amount of water it is given be y
If the amount a tree grows in one month is directly proportional to the amount of water it is given, then;
x ∝ y
x = ky
k is the constant of proportionality
k = x/y
Given
x = 8cm
y = 21 gallons
k = 8/21
k = 0.381cm/gallon