#### Answer

$$4\times 10^{18}$$

#### Work Step by Step

In scientific notation, the number of significant digits is the number of digits in the leading number between $1$ and $10$. The final answer should have the same number of significant digits as the least number of significant digits in any of the numbers involved in the multiplication:
$7.2\times 10^{11}$ has $2$ significant digits
$5\times 10^{6}$ has $1$ significant digit.
Therefore, the final answer should be rounded to have $1$ significant digit.
Rearrange factors and multiply:
$$(7.2\times 10^{11})(5\times 10^{6})=(7.2\times 5)(10^{11}\times 10^{6})$$
$$=4\times 10^{18}.$$