t = tens digit
u = ones (or units) digit

10t + u = the number

The sentence:

>>...the tens digit of a number is 6 more than the ones digit...<<

translated into algebra is

t = u + 6

So to get an expression for the number we start with

10t + u

and substitute (u + 6) for t, and get

10(u + 6) + u

Then we simplify using first the distributive principle to
get rid of the parantheses:

10u + 60 + u

Then we collect like terms 10u and u to get 11u, and the
final answer is

11u + 60

Edwin