#### Answer

$3.0\,m/s^{2}$

#### Work Step by Step

According to Newton's second law, $F=ma$
$\implies a=\frac{F}{m}=\frac{F}{m_{1}+m_{2}}=\frac{F}{\frac{F}{a_{1}}+\frac{F}{a_{2}}}$
$=\frac{1}{\frac{1}{a_{1}}+\frac{1}{a_{2}}}=\frac{a_{1}\times a_{2}}{a_{1}+a_{2}}=\frac{4.0\,m/s^{2}\times12\,m/s^{2}}{4.0\,m/s^{2}+12\,m/s^{2}}=3.0\,m/s^{2}$