I was looking at the problem under the link below (did not want to post the same problem twice) and I got confused by one of the comments. We have a random variable $X$ and then we have $C$ = $10$ + $20X$ + $4X^2$.
The first comment under says that $C$ = $10$ + $20X$ + $4X^2$ does not imply that $Var(C)$ = $400Var(X)$ + $16Var(X^2)$.
I am wondering what I am missing here since I did it the same way.