#### Answer

$2x$ and $3x$ cannot be added together as $3x$ has to be multiplied to $12-x$.

#### Work Step by Step

$2x$ and $3x$ cannot be added together as $3x$ has to be multiplied to $12-x$.
The correct solution is:
$=2x+(3x)(12)-(3x)(x)
\\=2x+36x-3x^2
\\=38x-3x^2
\\=-3x^2+38x$