I'd go for approximation. Making the approximation that n, n + 1 and n + 2 are all about equal to n, then you can simplify your equation.

I can't tell if the (n + 1) and (n + 2) are in the numerator or denominator. Is this actually n(n+1)(n+2) / 6?

If so, solve n\^3/6 = 100, work out what n is (approximately) and then try that value of n to see how well it works.