junior college 1 chevron_right H2 Maths

Hi I need help for this question
I tried using da/dt = da/dx x dx/dt
But I got 3 (3)^1/2 b instead of (3)^1/2 b
.
Why does da/dt =da/dx x dx/dt does not work
Why do i have to do da/dt= da/dh x dh/dx x dx/dt ?