Of course, each element of a matrix is a scalar, a real number--in computer science terms, a floating point number. The examples use integers so the arithmetic is easy. Here is another example, this time with floating point values.
Most spreadsheet programs and medium priced electronic calculators include matrix math functions. You might wish to investigate whatever you have. But for these exercises, try to do the work "in your head" in order to internalize the process.
"It is important to practice the ... procedure for multiplying matrices until it becomes automatic. Also, you should be able to pick out immediately the row of A and the column of B that combine to give a particular entry in AB."1
1. Ben Noble and James Daniel, Applied Linear Algebra, 3rd., Prentice-Hall, 1988.