Neural network partial differential method

hello, i'm triying to calculatie partial derivatives.

i make this equation

y = tansig(a+4b+3c)

but, i have a problem during the proof.

first, i think that del_y/del_b = derivate_tansig(4)

but it was wrong.

so, i want to calculate partial derivatives

like [ del_y/del_a del_y/del_b del_y/del_c].

how can i proof that del_y/del_b is 4?

if anyone knows the proof equation please help