Question

Solve 0= 2t^3-21t^2+ 40t

Answer 1

$0=2t^3-21^2+40t$
switch sides
$2t^3-21t^2+40t=0$
solve factoring 
$t(t-8)(2t-5)=0$
using the zero factor principle
$t=0$
solve
$t-8=0:t=8$
solve
$2t-5=0:t= \frac{5}{2}$
$t=0,t=8,t= \frac{5}{2}$
Hope this helps