An initial deposit of $200 is made into an account that pays 4.2% annual interest. what is a model to represent the balance in the account after x years?

You would use the equation y = A(1+r)^t 

A = Original amount = 200
r = interest rate = 0.042 (when converted from percent to decimal)
t = time = x 

Plug them in and you get the model: y = 200(1+0.042)^x 

or just simplify the parentheses to (1.042)