Question

Please help and explain

Answer 1

Comment
The two triangles (ABC and ADE) can be shown to be similar (which I'll do below). Their corresponding parts are also similar. 
Step One
Prove Similarity.
A is common to both triangles              Construction
<ABC = <ADE                                       Given as right angles.

By the AA Theorem both triangles are similar.

Step Two
Find x

AD is to AB as DE is to x
AD = 3 + 4 = 7
AB = 3
DE = 6
7 / 3 = 6/x Cross multiply
7x = 3 * 6
7x = 18 Divide by 7
x = 18/7
x = 2.571 <<<<< answer