To solve this problem, we first need to divide the expression (0,5y-1 2/3) by 2.1.

First, let's convert the mixed number 1 2/3 to an improper fraction:
1 2/3 = 3/3 + 2/3 = 5/3

So the expression becomes: 0.5y - 5/3.

Now, divide by 2.1:
(0.5y - 5/3) ÷ 2.1

To divide by 2.1, we can multiply by the reciprocal of 2.1, which is 1/2.1:

(0.5y - 5/3) * (1/2.1)

This simplifies to:

0.5y/2.1 - 5/3 * 1/2.1
0.2381y - 0.7937

So the answer is 0.2381y - 0.7937, or in decimal form, approximately 0.24y - 0.79.