Looking for a polygon with given diagonals...

The formula for getting the number of diagonals of a certain polygon is:

N (N-3) / 2

where n stands for the polygon..

But here's the question:

What kind of polygon has 54 diagonals?

I tried to manipulate the formula as in reciprocated it in this form:

54 divided by the difference of 54 and 3 then multiply the quotient by two

The answer I obtained is 2.11

Please help me...Please teach me how to manipulate a formula..I also tried to use the triangle method but it doesn't work...