<p>A block is fastened to a horizontal spring. The block is pulled to a distance x = 10cm&nbsp; from its equilibrium position (at x = 0) on a frictionless surface from rest. The energy of the block at x = 5 cm&nbsp; is 0.25 J&nbsp; . The spring constant of the spring is_________<span class="equation">\(Nm^{-1}\)</span>​​​​​​​</p>