F(x)=x^3-6px^2+9px what is the value of p? a- -1/3 b- 1/3 c- -3 d- 3

Astudent must have an average on five test that is greater than it equal to 80% but less than 90% to receive a final grade of b. devon's greades on the first four test were 78% 62% 91% and 80% what range if grades on the fifth test would give him a b in the course? ( assuming the highest grade is 100%)