Chapter 7 - Section 7.6 - Vectors: The Dot Product - 7.6 Problem Set - Page 411: 33

$(-4,-3)\cdot(6,-8)=-4(6)-3(-8)=0$

If the vectors are perpendicular, their dot product must be zero. Dot product$=(-4,-3)\cdot(6,-8)=-4(6)-3(-8)=-24+24=0$. Therefore, $-4\textbf {i}-3\textbf {j}$ and $6\textbf {i}-8\textbf {j}$ are perpendicular.