$mv_0 = (m+M)v_f \implies v_f = \dfrac{mv_0}{m+M}$

$KE = U_e \implies \dfrac{1}{2}(m+M)v_f^2 = \dfrac{1}{2}kA^2$

$\dfrac{1}{2}(m+M)\left(\dfrac{mv_0}{m+M}\right)^2 = \dfrac{1}{2}kA^2$

$\dfrac{m^2v_0^2}{m+M} = kA^2 \implies v_0^2 = \dfrac{(m+M)kA^2}{m^2} \implies v_0 = \sqrt{\dfrac{(m+M)kA^2}{m^2}}$

plugging the numbers into a calculator