How come $2 \times 3^k + 3^k = 3 \times 3^k$ I get something else
$$\begin{align*}
&2 \times 3^k + 3^k=\\
&2 \times 3^k \times 2=\\
&4 \times 3^k
\end{align*}$$
What does $3^k + 3^k$ give exactly?

Remember the order of precedence of operations. Here, 
First Multiply $\quad2\times  3^k\quad $  ... then add: $\quad(2\times 3^k) + 3^k\;$: 
$$2\times 3^k + 3^k = (2 \times 3^k) + 3^k = (3^k + 3^k) + 3^k = 3\times 3^k$$

Note that if we just let $x = 3^k$, then $$2 \times 3^k + 3^k = (2\times x) + x = (2 \times x) + (1\times x) =  (2 + 1)\times x = 3\times x.$$
Now, since we let $x = 3^k$, then $3 \times x = 3\times 3^k$.

For your last question, $3^k + 3^k = 2\times 3^k$, but note that the first operation to perform in your original question is $\quad(1st)$ "$\times$" $\quad$ then $\;\;(2nd)$ "$+$".
