___Rb + ___RbNO3 => ___Rb2O + ___N2
I would concentrate on balancing the most complex first. In this case, that is RbNO3.
Put a 2 in front of RbNO3 to balance the N2.
_Rb + 2RbNO3 => _Rb2O + 1N2
Doing that gives 6 Oxygen on the left, so put a 6 in front of the Rb2O on the right to compensate.
_Rb + 2RbNO3 => 6Rb2O + 1N2
There are 12 Rb on the right. 2 are covered in the 2RbNO3 so we put a 10 in front of the elemental Rubidium to finish.
10Rb + 2RbNO3 => 6Rb2O +N2