Concept:
If three numbers \( a, b, c \) are in an Arithmetic Progression (A.P.), then the middle term is the arithmetic mean of the other two:
\[ b = \frac{a + c}{2} \]
Step 1: Apply the A.P. property to the given numbers.
Given \( a = -5 \), \( b = k \), and \( c = -1 \).
\[ k = \frac{-5 + (-1)}{2} \]
Step 2: Simplify the expression.
\[ k = \frac{-6}{2} \]
\[ k = -3 \]