well there's a simple way to do this, you out each side by side like so and solve:
3x+6=-15+6x
then you move like terms to the other side (like terms have the same variables). So we'll add 15 to both sides to balance the equation out because you cant remove a number from one side without affecting the other so:
3x+21=6x
then because we don't want an equation equal to 0 we move 3x to the other side by substracting 3x from both sides:
21=3x
now we divide by 3 both sides and because 21 divided by 3 is 7:
