here initially before the recoil of nucleus the system is considered to be at rest
now by the conservation of momentum we can say
[tex]P_f = P_i[/tex]
[tex]m_1v_1 + m_2v_2 = 0[/tex]
now we know that
[tex]m_1 = 57 m_2[/tex]
also it is given that
speed of alpha nucleus is 3.4 * 10^5 m/s
[tex]v_2 = 3.4 * 10^5 m/s[/tex]
now by the momentum conservation equation we will have
[tex]57m_2* v_2 = -m_2*3.4*10^5[/tex]
[tex]v_2 = -\frac{3.4 * 10^5}{57} = -5.9 * 10^3 m/s[/tex]
so the recoil speed will be given as 5.9 * 10^3 m/s