Step 1

Given $ \large\frac{dx}{dt}=3$

$ \large\frac{dA}{dt}=?$ when $ x = b$

Step 2

$ A = \large\frac{1}{4} b \sqrt{4x^2-b^2}$

$ \Large\frac{dA}{dt}=\Large\frac{bx}{\sqrt{4x^2-b^2}}.\large\frac{dx}{dt}$

Ans $ \large\frac{dA}{dt} = \sqrt 3b$ when $ x = b$