一道英文数学题(微积分)

Let l denote the perimeter,a one side
(l-2a)/2*a=24,that is la-2a^2=48
differentiate (l-2a)/2*a=24 respect to the time t
a*dl/dt+l*da/dt-4a*da/dt=0
a=6,da/dt=0.3 insert we have (l=(24/6+6)*2=20)
6dl/dt+20*0.3-4*6*0.3=0
dl/dt=0.2 is the answe