Let u be the velocity with which the ball hits the ground, then\\\\(\\displaystyle { u }^{ 2 }=2gh\\\\)\\\\(\\displaystyle =2\ imes 9.8\ imes 10=196\\\\)\\\\(\\displaystyle \ herefore \\quad u=14 m/sec\\\\)If v be the velocity with which it rebounds, then\\\\(\\displaystyle { v }^{ 2 }=2\ imes 9.8\ imes 2.5=49\\\\)\\\\(\\displaystyle \\Rightarrow v=7m/sec\\\\)\\\\(\\displaystyle \ herefore \\quad \\Delta v=\\left( v-u \\right) \\\\)\\\\(\\displaystyle =\\left( 7m/sec \\right) -\\left( -14m/sec \\right) \\\\)\\\\(\\displaystyle =21m/sec\\\\)\\\\(\\displaystyle \ herefore \\quad a=\\frac { \\Delta v }{ \\Delta t } =\\frac { 21 }{ 0.01 } \\\\)\\\\(\\displaystyle =2100{ m }/{ { s }^{ 2 } }\\\\)