(A) Subtracting the right side from both sides, we get ...
... (20 -4x) -(12 -x +8 -3x) = 0
... (20 -12 -8) +x(-4 +1 +3) = 0
... 0 = 0 . . . . . true for all values of x (infinite solutions)
(B) Subtracting the right side from both sides, we get ...
... (5x +4x -3) -(24 +8x) = 0
... x(5 +4 -8) +(-3 -24) = 0
... x -27 = 0
... x = 27 . . . . . . (one solution)
(C) Subtracting the right side from both sides, we get ...
... (5x +6) -(2x +6 +3x -15) = 0
... x(5 -2 -3) +(6 -6 +15) = 0
... 15 = 0 . . . . . . . not true for any value of x (no solutions)
