Verified Answer :

I supposed π ≈ 3.14 and we have A = 490.87 km²:

Area(A) is:  A = π × r²  or   3.14 × r² = 490.87 →

 r² = 490.87/3.14 → r² = 156,32 → r = √156,32 → r = 12.5

But diameter (d) is:  d = 2 × r → d = 2 × 12.5 →d = 50 km