A manufacturer is in the business of producing small figurines. He finds that the daily cost in pesos,P, of producing n figurines is given by P= n2-120 +4200. How many figurines produced per day so that the cost will be minimum?

Answer 1

I will assume a typo and that you meant:
P= n^2-120n +4200

This is a parabola when graphed.
Both of your questions are answered once you find the vertex of that parabola.
What method have you learned to find that vertex ?