Answer
$1.458m/s^2$
Work Step by Step
We can find the required acceleration as follows:
$Net\space Force= 2Fcos35^{\circ}N-57N$
$Net \space Force=2\times 55cos35^{\circ}N-57N=33.106N$
The mass of the sled and the child $=19+3.7=22.7Kg$
Now $F=ma$
We plug in the known values to obtain:
$33.106N=22.7Kg\times a$
$\implies a=\frac{33.106N}{22.7Kg}$
$a=1.458m/s^2$
