Answer
$$ 6.768\times 10^{18}$$
Work Step by Step
We know that scientific notation is: $a*10^b$, where $a$ must be between 1 and 10. Thus, we first multiply the two numerical values and the two values raised to a power. If the value of $a$ is between 1 and 10, we are done. If it is not, we must change the value of $b$ such that the value of $a$ is between 1 and 10. Doing this, we find:
$$67.68\times 10^{17}\\ 6.768\times 10^{18}$$
