To solve the equations, we will isolate the variable x in each equation.

1) x - 317 = 123
To isolate x, we can add 317 to both sides of the equation:
x - 317 + 317 = 123 + 317
x = 440

2) 13 - x = -7
To isolate x, we can subtract 13 from both sides of the equation:
13 - x - 13 = -7 - 13
-x = -20
Since we have -x, we can multiply both sides of the equation by -1 to get x:
(-1)(-x) = (-1)(-20)
x = 20

3) (-21) - x = 19
To isolate x, we can add 21 to both sides of the equation:
(-21) - x + 21 = 19 + 21
-x = 40
Again, we can multiply both sides of the equation by -1 to get x:
(-1)(-x) = (-1)(40)
x = -40

Therefore, the solutions to the equations are:
x = 440, x = 20, and x = -40.