Answer
$10a^3+12a^2+9a-20$
Work Step by Step
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 2a^2+4a+5$
$\underline{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 5a-4}$
$\ \ \ \ \ \ \ \ -\ \ 8a^2-16a-20\longrightarrow$ multiply by -4
$\underline{10a^3+20a^2+25a\ \ \ \ \ \ \ \ \ }\longrightarrow$ multiply by 5a
$10a^3+12a^2+\ \ 9a-20\ \longrightarrow$ combine like terms
