[tex]\begin{gathered} P(x)=-0.05x^{2}+20x-500 \\ P(80)=780 \end{gathered}[/tex]
Explanation
give
[tex]\begin{gathered} Cost\text{ function C\lparen x\rparen=40x+500} \\ Revenue\text{ function R\lparen x\rparen=60x-0.05x}^2 \end{gathered}[/tex]
Step 1
find the profit function
[tex]\begin{gathered} P(x)=R(x)-C(x) \\ replace \\ P(x)=60x-0.05x^2-(40x+500) \\ break\text{ the parenthesis} \\ P(x)=60x-0.05x^2-40x-500 \\ add\text{ like terms} \\ P(x)=-0.05x^2+20x-500 \end{gathered}[/tex]
Step 2
now, evaluate for x=80
[tex]\begin{gathered} P(x)=20x-0.05x^2-500 \\ P(80)=-0.05(80)^2+20(80)-500 \\ P(80)=-320+1600-500 \\ P(80)=780 \end{gathered}[/tex]
so, the answer is
[tex]\begin{gathered} P(x)=-0.05x^{2}+20x-500 \\ P(80)=780 \end{gathered}[/tex]
I hope this helps you