The value of a for which the equations 3x−y+az=1, 2x+y+z=2, x+2y−az=−1 fail to have unique solution is __________

3x−y+az=1______12x+y+z=2−−−−−−−2x+2y−az=−1−−−−−−3Eq,1+2⇒5x+a+1z=3-----(4)2×(1)+(3)⇒7x+az=1------(5)Given system fail to have  unique solution.⇒  The system has no solution or infinitely many solutions⇒57=a+1a≠31⇒5a=7a+7∣⇒2a=-7⇒a=-72