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I do not see the figure of course so will assume all three are in contact along the x axis.
Total mass = 2 + 3 + 4 = 9 kg
it is all accelerated in the x direction at once
F = m a
18 = 9 a
a = 2 m/s^2
force between 1 and 2:
m = 7 kg
F = 7 * 2 = 14 Newtons
force between 2 and 3
m = 4 kg
F = 4 * 2 = 8 Newtons