a : b =6
=> a = 6b
(a-72) : (b : 3) = 6
inlocuim "a" cu "6b" in ecuatie
(6b - 72) x 3/b = 6 (se inverseaza fractia (b : 3) si impartirea devine inmultire
18 - 216/b = 6
216 /b= 6-18 => 216/b = 12 => b =216 : 12 => b = 18
a = 18 x 6 = 108
si facem verificarea
(108 - 72) : (18 : 3) =6
36 : 6 = 6 (ADEVARAT)