The cost, c, to produce b baseball bats per day is modeled by the function c (b)=0.06b^2-7.2b+390. what number of bars should be produced to keep costs at a minimum?

This function is a quadratic whose coefficients are a=0.06,  b= -7.2,  c = 390.

Because a is +, the graph is a parabola that opens up and has a minimum.  Your job is to find the value of b at this minimum (at the vertex).  It is

b = -(-7.2) / [2(0.06)], or b = 60.

Production costs will be minimized if the number of bats made is 60.