How many different possible structures (isomers) are there for the complex, [Mn(OH2)3(NH3)3] 3+? How many different possible structures (isomers) are there for the complex,
$\ce{[Mn(OH2)3(NH3)3]^{3+}}$?

I really don't know how to begin with this. I know the answer is 2, but i don't have the faintest idea how this is the case. Since $\ce{Mn}$ has a charge of $3+$, I know that its valence electron configuration is $3d^4$. I also know that $\ce{NH3}$ and $\ce{H2O}$ are monodentate, which means they contribute 1 lone pair.
Metal complexes with three each of two types of ligands $(\ce{MX3Y3})$ have two isomers: mer- and fac-isomers. For example, for $\ce{Co(NH3)3Cl3}$ there are two isomers:


  Fac for facial, in which all three ligands of each type are clustered on a single face of the octahedron and the three other three ligands are on the opposite face.





  Mer for meridional, in which the three ligands of each type are arranged meridianally in a row around the metal and the other three ligands are arranged likewise on the opposite side in a perpendicular meridian.