900
6 = 2×3
9 = 3²
15 = 3×5
lcm = 2×3²×5 = 90
to have a square number multiple of 90 we have to multiply by 2 and 5
2×3²×5 × 2×5 = 2²×3²×5² = (2×3×5)² = 30² = 900
Good luck !
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