You did it right. 1.35 + 7.00 and you have the right answer. is that what you mean?
The polyhedron has:
6 Faces, 8 vertices and 12 edges <u>in</u><u> </u><u>total</u><u>.</u>
But
3 Faces, 7 vertices and 7 edges <u>are</u><u> </u><u>visible</u><u>.</u>
15/35
3/7 the answer is 3/7 which is written in the lowest terms