Height for m months = 10 + 3m millimeters
You basically just have to plug in the number of months in the variable m.
So for 8 months
Height for 8 months = 10 + 3x8 millimeters
Height for 8 months = 10 + 24 millimeters
Height for 8 months = 34 millimeters
For 3 Years.
There are 12 months in 1 year.
For 3 years there are 12 x 3 = 36 months
Plug it into the equation
Height for 36 months = 10 + 3x36
Height for 36 months = 10 + 108
Height for 36 months = 118 millimeters