To write the equation for the bank account balance, we need to consider both the initial deposit and the monthly deposit.
For the bank:
- Initial deposit: $100
- Monthly deposit: $15
The balance \( y \) after \( x \) months can be expressed as:
\[ y = 15x + 100 \]
So, the equation for the bank account balance is:
\[ y = 15x + 100 \]