Which of the following is not a valid potential energy function for the spring force F= -kx?

Sasha has just gotten a new job in a nearby city. after comparison shopping, she found that renting a nice two-bedroom apartment would cost around $800 per month. her utilities would cost about $150 per month. sasha has enough money saved for a down payment, and she found that she can buy a three-bedroom house or condo with a mortgage payment of $1,000 per month, including taxes and homeowner's insurance. her utilities would cost about $200 per month. what is the advantage of buying the house over renting the apartment?