Part A:
Equation → 5 + x - 12 = x - 7
Combine Like Terms → -7 + x = x - 7
Subtract x to both sides → -7 + x - x = x - x - 7
Simplify → -7 = -7
Add 7 to both sides → -7 + 7 = -7 + 7
Simplify → 0 = 0
All real number are solutions.
Part B.
Using -4 for x.
5 + x - 12 = x - 7
5 + (-4) - 12 = -4 - 7
-11 = -11
Using 0 for x.
5 + x - 12 = x - 7
5 + 0 - 12 = 0 - 7
5 - 12 = -7
-7 = -7
Using 5 for x.
5 + x - 12 = x - 7
5 + 5 - 12 = 5 - 7
-2 = -2
Using all these numbers show that my original statement of "All real numbers are solutions" is correct.
Best of Luck!