For the function f(x)=(x^(2)+4x+(11/4)e^(x) I need to find where it is concave up.

So, I found the first derivative and then the second derivative which is,

f''(x)= e^(x)(x^(2)+8x+(51/4))

Then, I found the roots of the function using the quadratic formula which is (-8+/-sqrt(13))/(2)

I found it was concave upwards at the open interval: (((-8+sqrt(13))/2), inf)

But somehow this is incorrect. Can someone help me?