math
posted by marie .
If a polyhedron has 13 vertices and 21 faces, how many edges does it have?
I just need to know if I am doing this right.
v+f=e+2
13+21=34+2=36 edges.

correct.

No
v+f=e+2
e=v+f2=342=32
For a simple polyhedron F  E + V = 2. (This is also called the Euler's Formula.) 
Your equation is correct, but your answer is not. Your number of Edges is you e variable. so you are looking for the "e" what you found is "e+2"
As a general Rule with math equations, and you can do this before you put your numbers in, figure out what variable you will be looking for (in this case e=?) and move everything else to the other side of your equals sign. This will let you have an otomatic solution, because it will set you equation up as "e=..." which is exactly what you want to know, and thn you don't have the confusion that you had in this set up again.
so if your equation is v+f=e+2, e=?
then your next step is to get e alone on one side.
v+f=e+2
2= 2
v+f2=e
then plug your numbers in to get your answer.