misunderstanding of How to multiply delta with activation in back propagation NN

[zizuo2050]

د
I have a question in back propagation NN when compute for delta like
1 - 𝛿_3=(𝑎_3 - y)
2 - 𝛿_2=(Θ(2))𝑇𝛿_3∗𝑔′(𝑧(2))
3 - Δ(𝑙)=Δ(𝑙)+𝛿(𝑙+1)(𝑎(𝑙))(𝑇) my question is how to compute for delta triangle which delta_2 = 25 element and a_1 = 400 element