A necessary and sufficient condition for the pair of equations given in
(2.6) to have a common solution is that each equation individually have
a solution and -NM=A1U. A common solution X is given by (2.3)
above. Thus a solution X=X1=X2 of (1.1) is of the form (2.3) where
Y2 =N Mt and Y1= -MUt.