In order to express a small number (less than zero) in scientific notation we just have to move the decimal point until we reach the last digit, the exponent we use must be a negative number and it represents the number of times we moved the dot.
In this case, we initially had the number 0.216, as you can see, we have three digits on the right of the point, then we must move the dot 3 times, then the representation in the scientific notation of this number is:
0.216 = 216×10⁻³
