[tex]f(x) = - 3 {x}^{2} + 9x - 8 \\ = > f(5) = - 3 {(5)}^{2} + 9(5) - 8 \\ = - 3(25) + 45 - 8 \\ = - 75 + 45 - 8 \\ = - 38[/tex]
The answer is -38.