No special strategy!! Just the use of the formula.

Formula for determining the number of diagonals of a polygon: , with n being the number of sides

Since there are 6 times as many diagonals as numbers of sides, then number of diagonals = 6n

We now have: 



 ------ Cross-multiplying





n(n - 15) = 0

Number of sides, or        OR       n = 0 (ignore) 

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