A variable? A symbolic variable?
You define it as symbolic, then you use * to multiply. What is the problem? What did you try?
I would point out that almost certainly, this is going to be a bad idea, as computing a symbolic determinant of a 16x16 matrix will be very computationally nasty.
If you don't know how to multiply a variably times a matrix, this strongly suggests you need to read the getting started tutorials.
