They are algebraic symbols or known as unknown number. There is no true value for them. The value of algebraic symbols are based on equations
ie. 1+x = 2
    First, we isolate the x by canceling the +1 FROM BOTH SIDES(Balance method) by -1 ON BOTH SIDES.
   1+x-1 = 2-1
  So we are left with the x on the left side, and a single number on the right side. Therefore, our calculation is complete.
    x = 1
More complex cases would be
     2y-3 = 45
First, we need to isolate the algebraic symbol, which is 2y (yx2) by +3 ON BOTH SIDES 
     2y-3+3 = 45+3
            2y = 48
WE ARE NOT DONE! 2y=48 is still an equation. Therefore, to isolate the y, we need to divide BOTH SIDES by 2
          2y/2 = 48/2
               y = 24
There is no true or absolute or definite value for algebraic symbols like x,y,z,a,c,t,h,d etc. They are used to represent unknown numbers.
Hope this helped!