You are correct that these are both questions about arithmetic series. But your formulas are incorrect so your answers are wrong. You need a better undersanding of what each term of the formulas actually do. Anyway, here are the correct formulas:
In both problems, you know the sum as well as and . You can plug the first formula into the second to get rid of the , which would make the only variable. In solving the equation for , you should see that it is a quadratic equation: . The simpliest way to solve these will be by using the quadratic formula.
The answers I get are 20 months and 5 years.