Is there more to the question
Answer:
C. f(1)=6; f(n)=f(n-1)+6
Step-by-step explanation:
You can use Cymath. Just enter the equation
https://www.cymath.com
The C(x) equation is a parabola with a squared "x" and a positive leading term coefficient, meaning, is a vertical parabola and it opens upwards
the lowest point, or lowest cost value, it reaches, is the U-turn point, or vertex

so, the lowest cost is at