Hello, Shan.
To reply your query, please use this link to get a better idea of what and how it is taking place.
The layer_sizes function is applied to t_X and t_Y , **not to X and Y** .
Also, in exercise 4, W1 is already in transposed state. Although, in previous exercises, we had to transpose it first, but from week 3 onwards, the transpose is already done for you and you just have to apply it directly.
Thanks.