Help please....i am sticking here

(2)Newton's second law states that the magnitude of force acting on the body can be given as time rate of change of its momentum .
Or , the acceleration of the body is directly proportional to the external forces acting on it. 

(3) the tension of the rope can be calculated by analysing the system
$$\begin{gathered} first take the two trolleys in a system . it is clearly noted that \\ F=Ma \\ 46.5=(5.2+2)a \\ a=6.5m/s^2 \end{gathered}$$
Now the smaller trolley has free body diagram as 
So. 
$$\begin{gathered} T\cos 25-f\text{'}=m\text{'}a \\ T\cos 25-5.1=2*6.5 \\ T\cos 25=18.1N \\ T=20.0N \end{gathered}$$

Regards