What is the answer to 1-4a equals 21?

Answer 1

The answer ... usually called the "solution" ... is the number that 'a' must be
in order for the statement to be true. There's only one such number.

You said that [ 1 - 4a = 21 ].

I would begin by adding ' 4a ' to each side of the equation:
1 = 21 + 4a
Now subtract 21 from each side:
-20 = 4a
Finally, divide each side by 4 :
-5 = a