Step by step
The question
Question: multiply these two matrices.
[1, 2; 3, 4] × [1, 2, 3]
1 Check the sizes
A row of the first matrix has 2 entries, but a column of the second has 1.
They do not match up.
So we cannot multiply them: there is no answer.
2 ≠ 1: we cannot multiply
Answer
2 ≠ 1: we cannot multiply