(a) The force on 7th coin is due to weight of the three coins lying above it.
Therefore,
F = (3m)kgf = (3mg) N
where g is acceleration due to gravity . This force acts vertically downwards .
(b) The eighth coin is already under the weight of two coins above it and it has its own height too. Hence force on 7th coin due to 8th coin is sum of the two forces
i.e.
F = 2m + m = (3m) kg f = (3mg)N. The force acts vertically downwards .
(c ) The sixth coin is under the weight of four coins above it .
Reaction , R = - F = - 4m(kg) = -(4mgf)N Minus sign indicates that the reaction acts vertically upwards , opposite to the weight .