here's the problem, f(x)= s/1+s^2

I've been staring at this problem for almost 30 minutes and i can't see where i go wrong on my second derivative...i've already gotten my critical points for f'(x) which are -1 and 1

f'(x)= 1-s^2/(1+s)^2

f"(x)= 1-s^2/(1+s)^2 = (1+s)^2(-2s)-(1-s2)(2)(1+s)/(1+s)^4 ...cancel my (1+s) = (1+s)(-2s)-(1-s^2)(2)/(1+s)^3 =-2s-2s^2-2+2s^2/(1+s)^3 Ultimately getting -2(s+1)/(1+s)^3