You need to get the x's on one side of the equals and numbers on the other side. The best way to decide which side you want numbers and x's on is to choose the one that keeps the x's positive when they're collected together. In this case, the x's are best on the right because when we bring the x term from the left we get 10x-9x=x on the right, because transfers across the equals change - to + and + to -. When we transfer -9 from right to left it becomes +9 added to 6 makes 15. So we have 15=x so that's the same as x=15.