Frank A. answered 02/01/20
Knowledge, experience and insight from decades of teaching physics.
Since m1 is accelerating upwards at 0.53ms-2 (same as m2 down the incline), the net force exerted by the component of the weight of m2 along the incline must overcome the weight of m1 and then exert the additional force for m1 to accelerate.
∴ m2gsin12º=(9.81xm1+0.53m1)
or m2 = (9.81x5.4 + 0.53x5.4)/9.81x0.21
= 27kg (the answer should be stated at 2 sig figs )
EXPLANATION OF VECTOR COMPONENTS
Weight of m2, acting vertically downwards is the only actual relevant force in the second part of the problem.
m2g is resolved into rectangular components
The component along the incline is the vertical (or y-comp)
that is why the relevant force is taken as m2gsin12º