The function T(x) gives us the time required to reach a coma and x is the amount of carbon monoxide.
[tex]T(x)=0.0002x^2-0.316x+127.9[/tex]a) Set x=550 and solve for T as follows:
[tex]\begin{gathered} T(550)=0.0002(550)^2-0.316(550)+127.9 \\ \Rightarrow T(550)=60.5-173.8+127.9 \\ \Rightarrow T(550)=14.6 \end{gathered}[/tex]And remember that T is given in hours, then the answer to part a) is 14.6 hr.
b) Set T(x)=6 and solve for x as follows:
[tex]\begin{gathered} T(x)=6 \\ \Rightarrow0.0002x^2-0.316x+127.9=6 \\ \Rightarrow0.0002x^2-0.316x+121.9=0 \\ \Rightarrow x\approx669.2\text{ or }x\approx910.8 \end{gathered}[/tex]You can solve the quadratic equation by any method you want.
Remember that T(x) requires that 500<=x<=800, we cannot know the effect that 910.8 ppm can cause in a person. Then, the answer to part b) is x=669.2 ppm