Numbers 34:15
The two tribes and the half tribe have received their inheritance on this side Jordan near Jericho eastward, toward the sunrising.
Cross-reference
Numbers 32:32
Parallel
Numbers 32:32 reiterates the same agreement: the two and a half tribes receive their inheritance east of Jordan after fighting.
Numbers 32:19
Parallel
Numbers 32:19 states these tribes will not inherit west of Jordan because their inheritance is already east — directly explaining the basis for 34:15.
Numbers 22:1
Historical context
Numbers 22:1 locates Israel in the plains of Moab east of Jordan — the same region where these tribes settled.