The question you presented asks that you "[trim] off 5 percent of values from each end of x". The help file says that the trim argument is " the fraction (0 to 0.5) of observations to be trimmed from each end of x". How much will be trimmed off each end of x_mat[,9] by the code you presented:
mean(x_mat[, 9], trim = 0.1)
I am being somewhat indirect to try to fulfill the spirit of the forum's homework policy.