What is 7^2-13 and the explantion

If you shift the linear parent function, f(x)=x, up 13 units, what is the equation of the new function?

Ineed to know the answer to this algebra question.

Ebuka's monthly rent is $750. if ebuka pays the rent late, his landlord charges 4% interest per week that the payment is late. write a function that gives the total cost r(t), in dollars, of ebuka's rent if he pays it t weeks late.