Applying KVL to this circuit
According to your image ,Vy is equal to 14 volts. ( 21 - 7 = 14 ) You can't find Vx just by information in this image. The proof is you can write 3 independent equations, but you have 4 independent variables! If you know the voltage of node "1" or node that you marked it, you can find all of the voltages.
You should find more information in the book. Otherwise you can't solve this problem.
If you want to know that why you can't solve a system of three equations in the four variables you should search the related math books.