Answer : (a) $\;33.3$

Explanation :

Let the amplitude of S.H.M be A

The spring streches maximum by 2A

$2KA=f \quad $ (f friction force )

$f=0.5 \times 40 \times 10$

$2 \times 3 \times A=0.5 \times 40 \times 10$

$A\times 2 \times 3 =5 \times 20 $

$A=\large\frac{100}{3}=33.3 \;.$