Physics - NLM Question with Solution | TestHub

The block's speed is maximum where its acceleration along the incline is zero, i.e. spring force equals the component of weight along the incline.

The angle $\theta$ is measured from the vertical wall, so the incline makes $90^{\circ }-\theta$ with the horizontal and the weight component along the incline is $mg\cos \theta$.

$kx=mg\cos \theta \Rightarrow x=\frac{mg\cos \theta }{k}$.