input: Write a function to calculate the objective function of a linear equation in the form of ax + by = c. The function should return the value of the objective function based on the provided coefficients and constant. Use the following assert cases to test the function.
  assert linear_eq(3, 5, 7)==9
  assert linear_eq(2, 4, 6)==2
  assert linear_eq(1, 1, 1)==1
  assert linear_eq(0, 0, 0)==0
  assert linear_eq(10, 20, 30)==300
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
output: def linear_eq(a, b, c):
      return a*x + b*y - c