Finding x such that F(x) takes the maximum value   (see the 10 values of F between a and b -
generated in the second window - to decide when to stop the calculation, before rounding errors prevail).
Choose [a,b] where F has a bumb.   With "test" you find 10 values of F from a to b without starting the search for the maximum.
f(x) = h*x^4 + k*x^3 + p*x^2 + q*x + u
h k p q u
  a = b =