E) sqrt(30)
This is a nice problem. It was helpful to read the topic at the top.
Notice that the two right triangles are similar. Their sides are in proportion.
That is the side '3' is what 'x' is in the other triangle (you need to rotate it!)
Then 'x' is similar to '10'
3/x=x/10 ---> x^2=30, x=sqrt(30)
If you are patient, you can check that for sqrt(30) pythagoras theorem works for all the triangles.
Best
Then
