Let two equal sides of triangle  = a

Given base = b

Now area of triangle A = (1/2) * b*√{a² - (b² /4)}

Now differentiate with respect to x, we get

δA/δx = (b/2) * (1/2)* 2a/b*√{a² - (b² /4)} * δa/δx ............1

Given in the question

δa/δx = 3 and a = b

Put these values in equation 1, we get

δA/δx = (b/2) * (1/2)* [2b/[√{b² - (b² /4)}] *3

          = (3b² / 2)* /[√{(4b² - b² )/4} 

          = 3b² /√3b²

          = √3*√3b² /b√3

          = √3b cm/second  

So when the two equal sides are equal to the base, then area decrease = √3b cm/second