By b / A he is implying a horizontal fraction bar with b above and A below. Of course, since matrix multiplication is not commutative, it’s probably better to write something like /A * b (or Matlab uses the cute notation A \ b) instead of b / A, but the intention is fairly clear from context.

(In general, most of our mathematical notation conventions are designed for the real number system, and start falling over when we have “number-like objects” which behave a bit differently.)