There is an invisible 1 multiplied to the second binomial.
You are supposed to factor 2x(3x+5)-1(3x+5).
Since there is a shared (3x+5) in each subtracted part, that is your GCF, even though it is not what you are used to. So you divide (3x+5) out of both the subtracted parts and put it in front, in it's "undistributed" form.
This problem, factored, is (3x+5)(2x-1).
To check, you should see that if you distributed the whole (3x-5), it would multiply to 2x and also multiply to 1. That was what your original problem looks like. Remember, you are factoring (making it into a multiplication problem), not simplifying.