As given point M=(2a,5b). When point Q is at (4a,0):

$$Slope=\frac{y_2-y_1}{x_2-x_1}$$

$$Slope=\frac{0-5b}{4a-2a}=-\frac{5b}{2a}=-2.5$$

Similarly when point Q=(5a,0), then

$$Slope=\frac{0-5b}{5a-2a}=-\frac{5b}{3a}=-1.6$$

So it is clear that the slope will increase but never be positive.