since, the vector 8i+aj in the direction of the vector 4i−3j. That means both vectors are parallel to each other. By condition of parallel vectors, ‌
8
4
=‌
a
−3
⇒a=−6 Alternative Method Given also, ‌‌|8i+aj|=10 ⇒‌‌√64+a2=10 ⇒‌‌64+a2=100 ⇒‌‌a2=36 ⇒‌‌a=±6    (here we take (-) ve sign) ⇒‌‌a=−6