Question 1187308
R-C=Profit=-x^3+x^2+5x-1
Take the derivative and set it equal to 0.
-3x^2+2x+5=0=3x^2-2x-5=(3x-5)(x+1)=0

x= 5/3 as only positive root.

f(5/3)=(-125/27)+25/9+25/3-1=(1/27)(-125+75+225-27)=(148/27)=$5.48 thousands 
graph this
{{{graph(300,300,-10,10,-10,10,5.48,-x^3+x^2+5x-1)}}}