Why not start out by writing a linear relationship for the plant height?
h(m) = 10 + 3m, where h is the height in mm and m is the time in years, starting from zero (0).
After 8 months, the plant height is h(8) = 10 mm + 3(8) mm, or 34 mm.
After 3 years (36 months), the plant ht. is h(36) = 118 mm.