##4m/s^2[forward]##

Recall that Newton’s ##2^nd## Law is given by the formula:

##color(blue)(|bar(ul(color(white)(a/a)F_net=macolor(white)(a/a)|)))##

where:

##F_net=##sum of all forces acting on the object

##m=##mass

##a=##

For this problem we will set the positive directions to be east and down.

Start by breaking the variable ##m## into ##m_person+m_sled## since the acceleration of the person also depends on the mass of the sled.

##F_net=ma##

##F_net=(m_person+m_sled)a##

Assuming there is no friction involved the only acting force in the ##x## direction is the applied force. The forces acting in the ##y## direction are the normal and gravity forces. However when you add them together the force is equal to ##0N## so we will ignore them.

##a=F_app/(m_person+m_sled)##

Substitute your known values.

##a=(300N)/(60kg+15kg)##

##a=color(green)(|bar(ul(color(white)(a/a)4m/s^2color(white)(a/a)|)))##