Two springs one trolley, find velocity

[coconut62]

Homework Statement

Please refer to the image.

My workings= pencil
Answer= pen

Why is my working wrong? What wrong assumptions did I make?

Homework Equations

Hooke's law/Newton's second law/ Elastic potential energy =0.5 kx^2

The Attempt at a Solution

As shown in the image.

 

[Doc Al]

You used a kinematic formula for uniformly accelerated motion. That won't apply here, since the acceleration varies with displacement.

Stick to energy conservation.

What is the total spring PE at equilibrium?

What is the total spring PE when the trolley is displaced?

 

[coconut62]

Got it, thank you.