The Domain of an Algebraic Expression: Only addition and subtraction

To answer the question we first need to understand what X is.

X is an unknown, meaning it's a symbol that represents another number, an unknown one, that could be there in its place.

Usually in exercises we'll need to calculate and discover what X is appropriate for each exercise,

but in this case the result is given to us: X=8
Therefore, we can substitute (plug in) the value 8 everywhere X appears in the exercise.

So we get:

5*8-6

40-6
34

Usually we don't know the value of the unknown and need to find it,

However, in this case they give us a value, so the first action will be to substitute it into the expression,

Meaning, replace every place where X is written with 0.

5*0-6=
0-6=-6

Therefore, the result is -6.