input: Write a function that calculates the objective function of a linear equation. The function should take three parameters: a, b, and c, representing the coefficients of the linear equation ax + by = c. The function should return the value of the objective function for given values of x and y. If the linear equation is ax + by = c, the objective function should be a*x + b*y - c.
  
  
  assert linear_eq(3, 5, 7, 2, 1) == 6
  assert linear_eq(1, 2, 3, 4, 5) == 11
  assert linear_eq(2, 3, 4, 1, 2) == 6
  
  def linear_eq(a, b, c, x, y):
      return a*x + b*y - c
  
output: def linear_eq(a, b, c, x, y):
      return a*x + b*y - c