your work is fine, In presenting a solution you should state how you constructed part 1 (by similar triangles).
so we see a right triangle. it is divided into two parts, one is "x" and the other part that adds with "x" must be "5-x," correct?
so how do we find what "x" is?
POSSIBLE SOLUTION ?:
1. set the equation to 3/x = 4/5-x
2. cross multiply
3. get 7x=15
4. divide 7 on each side
5. x = 15/7
is this correct?