f(x) = (x-7)^2 + (x-4)^2 + (x-5)^2
Find x such that f(x) (cost function) is a minimum.

df/dx = 2(x-7) + 2(x-4) + 2(x-5) = 0
3x = 7+4+5
x = (7+4+5)/3

The biggest hurdle is the problem statement itself!