[math-fun] Mathematica problems

Can someone please explain why I got this error message: ----- In[40]:= Solve[a c e - a d h - b c h - b d e == a + c + e, b c e + a d e + a c h - b d h == b + d + h, Integers] Solve::ivar: b c e + a d e + a c h - b d h == b + d + h is not a valid variable. ----- ? Thanks, Dan