Convert 213base four to base5

Create a real-life situation and create a system and show how it might be modeled algebraically with a system of equations (or inequalities) show all work include a few sentences explaining the situation, your variables of choice and what each represents, and how you would set it up and solve it algebraically. !

Write two subtraction equations that are equivalent to 5 + 12 = 17.

Write the rational number in lowest terms 99/-15