what is 3.99 X 10^16 pls explain

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!