Try taking the derivative of a(x) = c(x)/x = 25000/x+100+4x^(1/2)
a maximum or minimum will usually occur at a critical point (that is, when a'(x) = 0),
so you should solve a'(x) = 0 for x, then determine whether a(x) achieves a minimum or maximum at that x value. To do this, you could use the first derivative or second derivative test.