It is known, from pre-calculus algebra, that a quadratic equation of the form
has a minimum point with x-coordinate
this problem, x = t and f = W, and W(t) is a quadratic function with
a = 0.03 > 0, b = -0.39, and c = 7.3.
and the minimum weight is
This, of course, is the result we obtained using the other methods.