i have this equation

f(x)= 80x + (5+4x)^2 - (2x^3/3)

i need to show that there is a non stationary point of inflection where the first derivative is maximized

the 2 x values i get are -3.14 and 19.14 although im not sure this is right

also for a non stationary point i am told the sign either sign of x doesnt vary when in my case it does???!!