Hmmm...there's 2 answers (positive and negative);
since you're apparently learning this stuff, I presume the positive answer
is what's expected; re-word your problem this way:
The difference of two numbers is 27.
The larger number is 1 less than 3 times the smaller.
What are the two numbers?
Let x = larger, y = smaller ; then:
x - y = 27 
x = 3y - 1 
Substitute  in : if you don't know what that means,
you need classroom help.