f(x) = (x2/2) - 3x - (1/2)
df(x)/dx = x - 3
Mean Value Theorem: You want to find a point c in the interval [a,b] such that:
df(c)/dx = (f(b)-f(a))/(b-a)
In the interval [4,6]:
c - 3 = (f(6)-f(4))/(6-4)
c - 3 = (0 - -4)/(6-4)
c - 3 = 2
c = 5