Remember when multiplying matrices the number of columns in the first matrix must equal the number of rows in the second matrix or they cannot be multiplied.
Remember the elements of the new matrix are formed after taking the sum of the product of each element of the row in the first matrix by each element of a column in the second matrix.
R1 = row 1 of matrix 1
C1 = column 1 of matrix 2
Multiply the first element of R1 by first element of C1 and add to the product of the second element of R1 by the second element of C1 etc until there are no more products to be found in R1C1
Ex: matrix 1 is 2 3
matrix 2 is 1 0
R1C1 = 2(1) + 3(2) = 8
8 is the first element in the new matrix
now R1C2 = 2(0) + 3(-3) = -9, second element of the new matrix