Question

The displacement of a block attached to a horizontal spring whose spring constant is 6 N/m is given by x = 0.4 \cos(4.1 t - 0.8) m. What is the earliest time (t > 0) when the kinetic energy equals one-half the potential energy?

*** I saw the answer to this question but I didn't understand why the following was done:

At the time when the potential energy equals to the half of it, the displacement of the block would be
x = radic (2*0.24/k) = 0.28 m.