Chapter 4 - Section 4.2 - Amplitude, Reflection, and Period - 4.2 Problem Set - Page 203: 12

$\frac{2}{5}$

We know that if $A$ is a real number and $B\gt0$, then the graphs of $y=Asin(Bx)$ and $y=Acos(Bx)$ will have an amplitude of $|A|$. Therefore, the amplitude of the graph of $y=-\frac{2}{5}sin x$ is $|-\frac{2}{5}|=\frac{2}{5}$.