Răspuns :
21 + -3x = 15.
21 + -21 + -3x = 15 + -21.
0 + -3x = 15 + -21.
-3x = 15 + -21.
-3x = -6.
x = 2.
21 + -21 + -3x = 15 + -21.
0 + -3x = 15 + -21.
-3x = 15 + -21.
-3x = -6.
x = 2.