input: Write a function to calculate the midpoint of a trapezoid.
  
  assert trapezoid_midpoint(15, 25, 35) == 20  
  assert trapezoid_midpoint(10, 20, 30) == 15  
  assert trapezoid_midpoint(6, 9, 4) == 7.5
output: def trapezoid_midpoint(a, b, c):
      return (a + b) / 2