Linear functions are said to be additive: $f(x + y) = f(x) + f(y)$
But if I have this simple function $f(x)= 7x+3$, I get, for example(at $x=5$ and $8$):
$f(5)=38$ and $f(8)= 59$. The sum is $97$.
$f(5+8)= 7\cdot 13+3 = 94$.
$94\ne 97$. How come? What did I miss?