Guided by Figure 1.3.13(a) in which the position vectors A, B, and C appear in black, red, and green, respectively, obtain vectors from B to A, and A to C.

The vectors $\mathbf{A}-\mathbf{B}$ and $\mathbf{C}-\mathbf{A}$, drawn in blue and gold, respectively, are both $\mathbf{i}\mathbf{-}2 \mathbf{j}-3 \mathbf{k}$, and hence, are collinear.

Thus, the tips of the vectors A, B, and C all lie along a straight line.

Tools≻Load Package: Student Multivariate Calculus

Enter vector A as per Table 1.1.1.
Context Panel: Assign to a Name≻A

Enter vector B as per Table 1.1.1.
Context Panel: Assign to a Name≻B

Enter vector C as per Table 1.1.1.
Context Panel: Assign to a Name≻C

Write the sequence of vectors $\mathbf{A}-\mathbf{B}\,\mathbf{C}-\mathbf{A}$ 
Press the Enter key.

Context Panel: Student Multivariate Calculus≻
Lines & Planes≻Angle

Load the Student MultivariateCalculus package.

Define the vectors A, B, and C.

Apply the Angle command.

Apply the DotProduct and Norm commands, as appropriate.