The length, $\bm{y}$, of a white whale was 162 centimeters (cm) when it was born and increased an average of 4.8 cm per month for the first 12 months after it was born. Which equation best represents this situation, where $\bm{x}$ is the number of months after the whale was born and $\bm{y}$ is the length, in cm, of the whale?