I see you have answered on the other post.
You said you had a trivial solution.
So why did you need to use Hectictar's picture.
Your answer is no more simple than mine or Hectictars.
You have just left out chunks to make it appear shorter.
https://web2.0calc.com/questions/geometry-help-thank-you_1#r4
The final answer is 3/5 and I apologize for contradicting you.
However:
You did not give a solution.
A solution requires working or reasoning and you have not given that.
You pretend that this question is trivial, which it is not.
My method is correct it is just probably not the easiest way to do it.
Hectictar has given a nice solution here: It starts much as mine does.
https://web2.0calc.com/questions/geometry-help-thank-you_1#r5
Here is a quicker solution again
Again I will let the side length be 2 units because most people would find that easier than calling it by a pronumeral and it makes no difference.
Using the cosine rule
\(2=5+5-2*\sqrt5\sqrt5cos\theta\\ 2=10-10cos\theta\\ 8=10cos\theta\\ cos\theta = \frac{4}{5}\\ \text{so by drawing a simple right angled triangle with this fact is is easy to determine that} \\ sin\theta=\frac{3}{5}\)
Now jugoslav would you like to enlighten us with your trivial solution?