It will take an additional 8 weeks to reach a height of 48 centimeters.
y = 4.5x
Since, the maximum height is 48 cm and the plant has already grown 12 cm, we deduct the 12 from 48 to get the additional weeks it needs to grow.
48 - 12 = 4.5x
36 = 4.5x
36/4.5 = 4.5x / 4.5
x = 8 weeks.