1789+987-21:(23•2)=

Let's solve this step by step:

1. First, let's solve the expression inside the parentheses: 23 • 2 = 46.
2. Next, divide 21 by 46: 21 ÷ 46 ≈ 0.4565.
3. Now, let's add the results: 1789 + 987 - 0.4565 ≈ 2775.5435

Therefore, the result is approximately 2775.5435.