How do you evaluate the function f(x)=x^3-2x^2+5x-8 for f(-5)?

Just replace the value of $x$ with $- 5$ in the formula of $f \left(x\right)$
$f \left(- 5\right) = {\left(- 5\right)}^{3} - 2 {\left(- 5\right)}^{2} + 5 \left(- 5\right) - 8$
$f \left(- 5\right) = - 125 - 50 - 25 - 8$
$f \left(- 5\right) = - 208$