Answer:
x = 9
Explanation:
The big triangle is similar to smaller one so we can use the side lengths' proportion to find x:
7/4 = (x-3)+8/8
7/4 = (x+5)/8 cross multiply expressions
56 = 4x + 20 subtract 20 from both sides
36 = 4x divide both sides by 4
9 = x