You can put this solution on YOUR website!
There is a method to do this that involves breaking down the matrix to 4 3X3 matrices, and then breaking down each of those into 3 2X2 matrices, yada, yada, yada. Big pain in the sit-down.
Go to http://people.richland.edu/james/lecture/m116/matrices/determinant.html if you are really interested in the manual method.
Put your numbers into a square array in Excel, one number per cell. Then go find an empty cell and type, assuming you put your numbers in cells A1 through D4: