Answer
$ \Delta E = 6000J$
Work Step by Step
The change in mechanical energy comes solely from the change in kinetic energy because the block is on a horizontal surface so the potential energy does not change.
$ \Delta E =\Delta K $
$ \Delta E = K_f - K_i $
$ \Delta E =\frac{1}{2}mv_f^2 - \frac{1}{2}mv_i^2 $
$ \Delta E = \frac{1}{2}m(v_f^2 -v_i^2) $
$ \Delta E = \frac{1}{2}(30^2m/s -10^2m/s) $
$ \Delta E = \frac{1}{2} \times15kg\times( 800m/s) $
$ \Delta E = 6000J$
