At a certain company, the monthly salary of project managers can be modeled by the function f(x) = x4 – 10x 2 + 10,000, where x is the number of years of employment. After how many years would a project manager be eligible for a $20,000 monthly salary? after working for exactly 10 years after working for at least 10 1/4 years after working for exactly 12 years after working for at least 12 1/4 years