a. Only a Hamiltonian path
One such path is
1 → 2 → 0 → 4 → 3
which satisfies the requirement that each vertex is visited exactly once.
There is no Hamiltonian circuit, however, since it is impossible for any Hamiltonian path on this graph to visit vertex 0 exactly once.
The answer is b.6, you set up a proportion of the corresponding sides in both triangles...12/9=8/x solve for x and get 6.

Explanation
Step 1
the density of an object is given by:

Step 2
let

Step 3
replace,

I hope this helps you
5a - 10*3 = 45
5a - 30 = 45
5a = 45 + 30
5a = 75
a = 15