input: Write a function to calculate the value of x given the values of a, b, and y.
  assert calculate_x(5, 7, 4) == -5.6
  assert calculate_x(10, 20, 5) == -10.0
  assert calculate_x(3, 9, 6) == -18.0
output: def calculate_x(a, b, y):
      return (y - b) / a