Use the polynomial remainder theorem. It says that a polynomial [tex]f(x)[/tex] has remainder [tex]f(c)[/tex] upon division by [tex]x-c[/tex].
Here we have
[tex]f(x)=x^4+3x^3-8x^2+5x-9[/tex]
and [tex]c=-5[/tex], so the remainder is
[tex]f(-5)=(-5)^4+3(-5)^3-8(-5)^2+5(-5)-9=\boxed{16}[/tex]
