How do you multiply this set of matrices?

[5  4  8]  \/  {-8  6}               
[3 -4 -2]  /\  { 0 -2}

The first matrix is a 2x3 and the second one is a 2x2.

You can only multiply an n×m matrix on the left by an
m×p matrix on the right.

The inner two dimensions must match or you can't
multiply them:

Examples:

A 5×8 matrix on the left CAN be multiplied by an 8×4 
matrix on the right because the two inner dimensions
are both the same, namely 8.

An 8×4 matrix on the left CANNOT be multiplied by an 5×8 
matrix on the right because the two inner dimensions
are not the same, one being 4 and the other 5.

A 5×8 matrix on the left CAN be multiplied by an 8×4 
matrix on the right because the two inner dimensions
are both the same, namely 8.

An 6×4 matrix on the left CAN be multiplied by an 4×1 
matrix on the right because the two inner dimensions
are both the same, namely 4.

In your case, you CANNOT multiply them because you
have a 2×3 on the left and a 2×2 matrix on the
right, and the inner dimensions, 3 and 2, do not match.

Now if you had those two matrices reversed, you
COULD multiply them, for then you'd have a 2×2 matrix
on the left and a 2×3 on the right and the inner 
dimensions would both equal 2. 

[-8  6]  \/  {5  4  8}               
[ 0 -2]  /\  {3 -4 -2}

then the answer would have been this 2×3 matrix

[-22 -56 -76]
[ -6   8   4]

If you need to learn how to multiply two matrices
when they CAN be multiplied, repost asking how.

Edwin