Represent the following sentence as an algebraic expression, where "a number" is the letter x.

Eight times the difference of 5 and a number.

By the difference of "5 and a number", I assume you mean that number minus 5. In that case, the expression is 8 times (x-5).

If by "the difference of 5 and a number" you mean 5 minus that number, then the expression is 8 times (5-x).