The amplitude of a cosine function of the form \( j(x) = A \cos(x) \) is given by the absolute value of the coefficient \( A \).
In your function, \( j(x) = -\frac{2}{7} \cos(x) \), the coefficient \( A \) is \( -\frac{2}{7} \).
The amplitude is calculated as:
\[ \text{Amplitude} = |A| = \left| -\frac{2}{7} \right| = \frac{2}{7} \]
Therefore, the amplitude of \( j(x) = -\frac{2}{7} \cos(x) \) is
\[ \frac{2}{7}. \]