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) == (3*x + 5*y - 7)
  
output: def linear_eq(a, b, c):
      return a * x + b * y - c