input: def linear_eq(a, b, c):
      """ Calculate the objective function of a linear equation ax + by = c using given values of a, b and c.
      
      >>> linear_eq(3, 5, 7)
      -2
      """
      return a*x + b*y - c
output:     x = 1
      y = 1
      while a*x + b*y != c:
          if a*x + b*y > c:
              y -= 1
          else:
              x += 1
      return x