svetazuna 40 - 3х = 10,
-3х = 10 - 40,
-3х = -30,
х = -30 : (-3),
х = 10,

проверка:
40 - 3*10 = 10,
40 - 30 = 10,
10 = 10

BableBi -3x=10-40
-3x=-30
-x=-10
x=10