Since they are of different base, convert to base 10

  243\(_{six}\) = (2 x 62) + (4 x 61) + (3 x 60)

  = 72 + 24 + 3 = 99 base 10

  243\(_{six}\) = 2 x 52 + 4 x 51 +3 x 50

  50 + 20 + 3 = 73 base 10

  Subtracting them, 99 - 73

  = 26