.9X=15-.6X
Add 0.6 X to both sides.
That will give you the valid equation
1.5 X = 15
Next, divide both sides of the equation by 1.5. Then you will know the value of X.
That will give you the valid equation
1.5 X = 15
Next, divide both sides of the equation by 1.5. Then you will know the value of X.