**So far, we've learned how to add and subtract matrices (the sizes had to be the same) and how to multiply a matrix by a scalar. These were really easy.**

**Multiplying two matrices is a bit tricky, but, once you get the hang of it, it's a snap -- you just need to be careful!**

**First of all, the size of the two matrices you are multiplying is super important!**

**Here's how it works:**

**Say the size of matrix A is 3 x 4and the size of matrix B is 4 x 1and we want ...**

**Looking at the sizes...**

**You'll see this when we do one.**

**In general:**

**If the size of matrix A is m x nand the size of matrix B is n x p,**

**then**

**This is for ...**

**Can we multiply ? No!**

**For this to all make sense, we really just need to do one.**