Energy conservation in oscillatory motion

[mansfief]

Homework Statement

A 0.321-kg mass is attached to a spring with a force constant of 13.3 N/m. If the mass is displaced .256 m grom the equilibrium and released, what is its speed when it is .128 m from equilibrium. The answer is 1.43 m/s

Homework Equations

k=(w^2)m
w=2pi/T

The Attempt at a Solution

I used the formula (1/2)mv^2= (1/2)kd^2. I solved for k and got 6.44 rad/s and then i solved the equation for v. I then used .128 for my d. I plugged in my answer and got 8.24. I also used d as .256 and got 1.64

 

[cupid.callin]

k is given as 13.3

here's how energy conservation works

for one state of system ... write all mechanical (potential + kinetic) energies associated with it
the do same for other state
the 2 energies shall be same so equate them and find unknown

so here find energy when spring is initially stretched and when it has extension .128 and equate them