input: Write a function to simulate the objective function of a linear equation ax + by = c. The function should take three arguments a, b, and c and return the expression a*x + b*y - c. Include assert statements to test the function.
  assert linear_eq(3, 5, 7) == 3*x + 5*y - 7
  assert linear_eq(1, 2, 3) == x + 2*y - 3
  assert linear_eq(4, -1, 6) == 4*x - y - 6
  assert linear_eq(0, 0, 0) == 0 - 0
output: def linear_eq(a, b, c):
      return a*x + b*y - c