To solve this expression, we need to follow the order of operations (parentheses, exponents, multiplication and division from left to right, and addition and subtraction from left to right).

First, let's simplify the expression inside the parentheses:

2 1/2 + 1 3/4
= (2 + 1) + (1/2 + 3/4)
= 3 + (2/4 + 3/4)
= 3 + 5/4
= 3 1/4

Now we have:

8 1/2 - 3 1/2 : (3 1/4) • 2 3/7

Next, let's divide:

8 1/2 - 3 1/2 / (3 1/4) • 2 3/7
= 8 1/2 - 3 1/2 / (13/4) • (17/7)
= 17/2 - 7/2 / (13/4) • (17/7)
= 10/2 / (13/4) • (17/7)
= 5 / (13/4) • (17/7)
= 5 / (13/4) • (17/7)
= 5 / (13/4 17/7)
= 5 / (221/28)
= 5 28 / 221
= 140 / 221

Therefore, the final answer is 140/221.