Matrix, Dimensions.
for the sum of these two things that I have in the boxes to be itself a 5 by 1 vector. The more general rule is that in the example on the left, b^[1] is n^[1] by 1, like this 3 by 1. In the second example, it is this is n^[2] by 1 and so the more general case is that b^[l] should be n^[l] by 1