2. If the vector \( a =3 \hat{ j }+4 \hat{ k } \) is the sum of two vectors \( a _{1} \) and \( a _{2} \), vector \( a _{1} \) is parallel to \( b =\hat{ i }+\hat{ j } \) and vector \( a _{2} \) is perpendicular to \( b \),
[3 May 2019, Shift-1] then \( a _{1}= \)
