fix sign error

src/3_drude_model_solutions.md

@@ -74,7 +74,7 @@ $$
 $$
 
 $$
-\frac{dv_y}{dt} = \frac{eBv_x}{m} + \frac{eE}{m}
+\frac{dv_y}{dt} = \frac{eBv_x}{m} - \frac{eE}{m}
 $$
 
 Repeat steps followed in 2. to compute x(t) and y(t) after which the particle motion is given by,