input: Write a function to calculate the objective function of a linear equation of the form ax + by = c. The function should take three parameters a, b, and c and return the result of ax + by - c.
  assert linear_eq(3, 5, 7) == -7
  assert linear_eq(1, 2, 3) == -3
  assert linear_eq(4, 4, 8) == -8
  assert linear_eq(2, 1, 5) == -5
  assert linear_eq(0, 0, 0) == 0
output: def linear_eq(a, b, c):
      return a * x + b * y - c