why might function notation be important? the reasons notation is used?

Question

Once you get out of three dimensional functions, it is hard to write it any other way. Example:

f(x,y,z,t, u, v)=2x +y*e^t + uv -3zuv²

Answer 1

to rent a dvd a customer must pay $3.99 plus $0.99 for every day that it is late, write a rule in function notation for this situation?

f(x)=3.99+0.99x