\(F = m \omega^{2} A =\mu mg\)
So,
\(A =\frac{\mu g }{\omega^{2}}\) And \(\omega=\sqrt{\frac{ K }{ m }}\)
So,
\(\omega=\sqrt{\frac{100}{1.5}}\)
Substitute the values.
\(A =\frac{(0.4)(9.8)}{\left(\sqrt{\frac{100}{1.5}}\right)^{2}}\)
\(=6\, cm\)