The vector P→=ai^-2aj^+3k^ and Q=ai^+j^−k^ are perpendicular to each other, the negative value of a is

When P¯ and Q¯ are perpendicular to each other, their dot product will be zero P¯.Q¯=0 ⇒a2−2a−3=0 ⇒(a−3)(a+1)=0 ∴a=3 or −1