Answer
2.178 x 10^-7
Step-by-step explanation:
You can only have one number to the left of the decimal. 21.78 x 10^-6 has 2 numbers to the left of the decimal. To make it standard form move the decimal left one, and add that value to the -6, in this case that would add a -1. That makes it 2.178 x 10^-7.
