If the angle between line with d.c’s  −221,a21,b21 and other line with d.c’s  354,354,−654 is 900 then a pair of possible values of  ‘a’  and ‘b’ respectively are

−2212+a212+b212=1(∵l2+m2+n2=1) 4+a2+b2=21 ⇒a2+b2=17.......(1)Angle between the given lines is 900⇒−221354+a21354+b21−654=0−6+3a−6b=0⇒3a−6b=6⇒a−2b=2.....(2)Solving (1) & (2), we get a possible solution given by a=4:b=1