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)
1.) x=12/k.
2.) x=k/9+2c/9.
Hope that helps!