The answer is:
[tex]6.561*10^{-7}\\0.0000006561[/tex]
The explanation is shown below:
1. You have the following expression given in the problem above:
[tex](8.1*10^{-4})^2[/tex]
2. By applying the properties of exponents, the power rule, you can multiply the power 2 by and the power -4, then:
[tex]65.61*10^{-8}=6.561*10^{-7}[/tex]
3. To write as a decimal, you must move the decimal point 7 places to the left. Therefore, you have:
[tex]0.0000006561[/tex]