#### Answer

$9$ players

#### Work Step by Step

Plug the given number of games into the formula. Then multiply by the denominator on both sides and put all terms on one side. Then solve by finding two $x$-values that equal the third term when multiplied and add up to the coefficient of the second term.
$N=\frac{x^2-x}{2}$
$36=\frac{x^2-x}{2}$
$72=x^2-x$
$x^2-x-72=0$
$(x+8)(x-9)=0$
$x=-8,9$
$x=9$
Only the positive value will matter since the question is asking for the number of players who were entered in the tournament.