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