Since, the angle between the vectors a and b is acute and the angle between b and y -axis lies between
Ï€
2
and π, it means they are obtuse ∴a⋅b>0‌and‌b⋅j<0 ⇒(xi−3j−k)⋅(2xi+xj−k)>0 and (2x˙i+xj−k)⋅j<0 ⇒2x2−3x−1>0 and x<0 ⇒(2x−1)(x−1)>0 and x<0