input: Write a function to calculate the objective function of a linear equation. The function should take three parameters a, b, and c and return the result of the equation ax + by - c.
  assert linear_eq(3, 5, 7)==0
  assert linear_eq(2, 4, 6)==0
  assert linear_eq(1, 8, 9)==0
  assert linear_eq(6, 3, 9)==0
  assert linear_eq(0, 0, 0)==0
output: def linear_eq(a, b, c):
      return a * x + b * y - c