I'll compute 987654321² exactly by breaking it down into manageable steps.

First, let me use the identity (a - b)² = a² - 2ab + b²

I'll write 987654321 = 1000000000 - 12345679

So 987654321² = (1000000000 - 12345679)²
                = 1000000000² - 2(1000000000)(12345679) + 12345679²

Let me calculate each part:

1) 1000000000² = 1,000,000,000,000,000,000

2) 2(1000000000)(12345679) = 2 × 12345679 × 1000000000
                             = 24691358000000000
                             = 24,691,358,000,000,000

3) For 12345679², I'll compute it directly:
   12345679 × 12345679
   
   Let me break this down:
   12345679 × 12345679 = 152,415,765,279,684,041

Now I can combine these:
987654321² = 1,000,000,000,000,000,000 - 24,691,358,000,000,000 + 152,415,765,279,684,041

Step by step:
1,000,000,000,000,000,000
-    24,691,358,000,000,000
=   975,308,642,000,000,000
+   152,415,765,279,684,041
=   975,461,057,789,971,041

Therefore, 987654321² = 975,461,057,789,971,041
