3x + 6 = 12
First operation was subtraction (subtracting 6 to leave 3x alone)
Second operation was division (Dividing 3 from 3x to isolare x)
3x + 6 = 12
3x + 6 -(6) = 12 - 6
3x = 6
[tex] \frac{3x}{3} [/tex] = [tex] \frac{6}{3} [/tex]
3 and 3 cancels out
x = 2