Draw a right angled triangle with height 3, base x and hypotenuse (that is line from observer to plane) making angle A with the base. If the plane is approaching observer dx/dt=-400 We are trying to find dA/dt
Now dA/dt=dx/dt times dA/dx
From the diagram tanA=3/x So x=3cotA dx/dA=-3cosecAcotA So dA/dx=1/(-3cosecAcotA)=-1/3sinAtanA
So dA/dt=-400times -1/3sinAtanA
When x=4 tanA=3/4 and using Pythagoras sinA=3/5 Put in these values to work out dA/dt
If plane is flying away from observer use dx/dt=+400 instead.