Answer:
a = 1.458 [m/s²]
Step-by-step explanation:
To be able to solve this type of problems it is always advisable to make a free body diagram with the forces acting on the body.
Attached is an image with the free body diagram and the proposed equations, the explanation of obtaining the equations will be described below.
Let's take the movement to the left as positive, i.e. all forces going to the left have a positive sign.
Let's use Newton's second law, which tells us that the sum of forces on a body is equal to the product of mass by acceleration.
To the left we have the two forces of tension by the cosine of the angle. And to the right the frictional force. The resultant of these forces is equal to the product of the mass (mass of the child plus the mass of the sled), by acceleration.