18n+[n(n-1)/2]*(-3)=54
36n-3n(n-1)=108
36n-3n^2+3n=108
-3n^2+39n=108
3n^2-39n=-108
n^2-13n=-36
n^2-13n+169/4=-36+ 169/4
(n- 13/2 )^2=25/4
n- 13/2 =±5/2
n=±5/2+ 13/2
n=9或n=4