Let's start by defining our variables:
W = weight
M = months since birth
We know that the baby weights about 8 pounds at birth, so this is our starting value
W = 8
We also know that every month this increases by 1.5 pounds every month since birth.
W = 8 + 1.5M
So our equation that relates weigh in pounds to age in months is
W = 1.5M + 8