First substitute the value of x
Then transpose the value of x which is 225 to the left hand side. Don't forget the sign when you're transposing. :)
Since the missing value is +K and not -K. Multiply both sides by negative 1 :)
You can also check your answer by substituting the value of x and the value of k.
Yay. The answer is correct if the value you've solved is equal to the equated value. I hope I could help. :)