f(t) = (1/t)et/6
f'(t) = (1/6t)et/6- (1/t2)et/6
0 = (1/6t)et/6- (1/t2)et/6
0= et/6(1/6t - 1/t2)
0 = t - 6
t = 6
When t<6, the slope of the function is negative meaning that the function is decreasing. When t>6, the slope is positive which means the function is increasing. This means the function has a local minimum at t=6