The equation is just like the y = mx + b rule, except written in a different order and with different variables.
Knowing this, find how fast the baby is growing each month, or its growth rate.
Using the hint from part (a), find the length when the baby was born, or its starting value.
Substitute 3 for the time, or the variable t, and solve for the length.
First substitute 39.5 for the length and solve for the age of the baby.
Then, add that many months to January to find in what month the baby will be 39.5 inches long.