Answer
$$-7.$$
Work Step by Step
$$(-k)\cdot (i-2j+7k)=-k\cdot i+2k\cdot j-7k\cdot k
=-7.$$
Recall that:
$i\cdot j=j\cdot i=k\cdot j=j\cdot k=i\cdot k=k\cdot i=0$
These dot products are zero because the vectors are perpendicular ($\theta=90^{\circ}$ and $\cos \theta=0$).
Recall that:
$i\cdot i=j\cdot j=k\cdot k=1$
These dot products are 1 because the vectors are parallel ($\theta=0^{\circ}$ and $\cos \theta=1$) and their length is 1 (unit vectors).
