tan(α)=3x⟹dαdx=−39+x2tan(α)=3x⟹dαdx=−39+x2tan(α+θ)=13x⟹dθdx+dαdx=−13169+x2tan(α+θ)=13x⟹dθdx+dαdx=−13169+x2dθdx=−13169+x2+39+but the actual answer is you have to use this formula (with respect to tt), not an xx derivative. So you'll have to take into account that dxdt=3dxdt=3 ft/sec