First, notice that the expression for f(f(x)) is the following:
[tex]f(f(x))=(x^2+ax+b)^2+a(x^2+ax+b)+b[/tex]notice that the first term is a quadratic expression with exponent 2. This means that f(f(x)) has 4 zeros.
Since f(x) has 2 zeros (since its quadratic), we have that M-m = 4-2 = 2, thus, M-m will never be 0, 1 or 3
