Newton's third law says:
"For every action, there is an equal and opposite reaction. ".
So, the force that Tom does on the sister is equal to force the sister applies on Tom:
[tex]F_t = F_s[/tex]
where the label "t" means "on Tom", while the label "s" means "on the sister".
From Newton's second law, we also know
[tex]F=ma[/tex]
where m is the mass and a the acceleration. so we can rewrite the first equation as
[tex]m_t a_t = m_s a_s[/tex]
And find Tom's acceleration:
[tex]a_t = \frac{m_s}{m_t} a_s = \frac{15 kg}{61 kg} (2.1 m/s^2) =0.52 m/s^2[/tex]
