Chapter 3.2, Problem 1P

The first equation in (2.6) written out in detail is

$M_{11x1}+M_{12x2}+M_{13x3}=k_1$

Write out the other two equations in the same way and then substitute, $x_1,x_2,x_3=x,y,z$ and the values of $M_{ij}$ and $k_i$ from (2.4) and (2.5) to verify that (2.6) is really (2.3).