Please solve

Uh oh! Squaring a binomial does not work that way!

The error was made at the step

$\left(\sqrt{2}+\sqrt{8}\right)^2\Rightarrow\sqrt{2}^2+\sqrt{8}^2$

This is incorrect.

$(a+b)^2\neq a^2+b^2$

How do I know that? Well, if you are ever unsure, try doing it yourself.

$(a+b)^2=(a+b)(a+b)=a(a+b)+b(a+b)=a^2+ab+ab+b^2=a^2+2ab+b^2\neq a^2+b^2$

Therefore, when you square a binomial, $(a+b)^2=a^2+2ab+b^2$.

Therefore, you can do the following:
 

Oh! Thank you! Well, I see where I went wrong, so thank you! Good job answering their question properly.