Put parenthese around 10^16 first.
So, the question would look like this.
$${\mathtt{3.99}}{\mathtt{\,\times\,}}\left({{\mathtt{10}}}^{{\mathtt{16}}}\right)$$
Now, find the answer to the exponent.
$${{\mathtt{10}}}^{{\mathtt{16}}} = {\mathtt{10\,000\,000\,000\,000\,000}}$$
So, now you multiply.$${\mathtt{10\,000\,000\,000\,000\,000}}{\mathtt{\,\times\,}}{\mathtt{3.99}} = {\mathtt{39\,900\,000\,000\,000\,000}}$$
So, $${\mathtt{39\,900\,000\,000\,000\,000}}$$ is our answer.
Hope it helps!