Answer:
![3x+12=57](https://img.qammunity.org/2020/formulas/mathematics/middle-school/snv83ar2x41jndvkusxnkr35bzvndrqmc2.png)
Explanation:
Let x represent number of days.
We have been given that Marco earns $3 per day working. So amount earned in x days would be
.
We are also told that Marco already has $12 saved, so total amount saved by Marco in x days would be
.
Now we will equate total amount saved by Marco in x days with 57 as Marco needs to save $57 to buy new shoes.
![3x+12=57](https://img.qammunity.org/2020/formulas/mathematics/middle-school/snv83ar2x41jndvkusxnkr35bzvndrqmc2.png)
Therefore, the equation
shows the number of days (x) that Marco must work before he can afford the shoes.