Let $n=2m$, where $m$ is odd. How many elements of order $2$ does the group $D_n/Z(D_n)$ have? 
Let $n=2m$, where $m$ is odd. How many elements of order $2$ does the group $D_n/Z(D_n)$ have?

I don't how to begin this proof. All I have so far is that Dn/Z(Dn) should have one element of order 2.

SO I have that the Dn/Z(Dn)={R0,R180}. I also figured out that the order of Dn=(m+1)(n+1)=(mn)+2n+m+1. But I don't know where to go from here.
