Similarly, we find
$$ \frac{\partial C}{\partial w_{21}^{(1)}}=\delta_2^{(1)}a_1^{(1)}, $$and
$$ \frac{\partial C}{\partial w_{22}^{(1)}}=\delta_2^{(1)}a_2^{(1)}, $$where we have defined
$$ \delta_2^{(1)}=w_2^{(2)}\frac{\partial a_2^{(1)}}{\partial z_2^{(1)}}\delta^{(2)}. $$