Find All Fixed Points for the Function by Setting the Function

Multiple Choice

Find all fixed points for the function by setting the function equal to x and solving the resulting equation for x.
-Use Gauss's function, $f ( n ) = \frac { 1 } { 2 } n ^ { 2 } + \frac { 1 } { 2 } n$ , to find a natural number $n$ such that the sum of the first $n$ natural numbers is 378 .

A) 27
B) 29
C) 28
D) 26