Question 805402
210 = n^2 + n
0 = n^2 + n-210
 n^2 + n-210=0
If you are lucky, you can factor or complete the square
(n+15)*(n-14) =0
n=-15 and n=14 are solutions

The method that always works is the quadratic equation. see below

*[invoke solve_quadratic_equation 1, 1, -210]