input: Write a python function to calculate the final grade based on two exams and homework scores.
  assert grade_calculator(95, 80, 100) == 91.0
  assert grade_calculator(70, 85, 90) == 79.5
  assert grade_calculator(50, 60, 70) == 57.0
output: def grade_calculator(exam1, exam2, homework):
      return 0.3 * exam1 + 0.3 * exam2 + 0.4 * homework