input: Write a python function to solve a given mathematical expression using the eval function.
  assert expression_solver("2 + 2") == 4
  assert expression_solver("10 - 3") == 7
  assert expression_solver("5 * 6") == 30
  assert expression_solver("8 / 2") == 4.0
output: def expression_solver(expression):
      return eval(expression)