an algebraic expression that describes a two digit number whose units digit is 7 less than its tens digit

The expression is

10n + (n-7)

where ' n ' is any digit.

Y'know, I guess you could just call it (11n - 7).

As long as you're careful to make ' n ' a digit, this expression should come out to be a 2-digit number with the units digit being 7 less than the tens digit.