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