That's probably a little technical. On a benzene ring with a single substituent, we call the position with the substitution the 1-position. So, we then have the two positions next to the 1-position, 2- and 6-, these are the ortho positions. The next two positions around, 3- and 5- are the meta positions, and the position directly opposite the 1-position is the para, or 4- position (so you can now make sense of names like paramethoxyamphetamine and metachlorophenylpiperazine). Different substituents direct further substitutions to different positions- some groups, for instance alkyl or alkoxyl, tend to direct further substituents to either an ortho or a para position. Now, the 1-,2-,and 5-positions are already occupied by the ethylamino and two methoxy groups, respectively. As cortex says, this means the 3-position is unfavourable. We're now left with the 4- and 6- positions, and it could go either way, except that at the 6- there is steric hindrance (or, in layman's terms, too much shit in the way already). So, para it is. This is probably more suited to the B&D Not-Quite-Advanced questions thread, and if you are hoping to learn a little more drug chemistry, reading through, and posting in, that thread is a really good way to go about it. If you already understood o/m/p notation, then excuse the condescension.
