
first of all you need to multiply by 5 both sides if the equation. Then you get
5a=2x+6
now move 2x to the left side and 5a to the right side. Don't forget, when you move numbers like that you need to change their sign (if there were 5a it becomes -5a)
-2x=6-5a (now divide both sides of the equation by -2)

or
x=2.5a - 3