The net force is 59 N but the acceleration is 0.017 ms-2.
The term resultant force refers to that force that has the same effect in magnitude and direction as two or more forces acting together. We can talk of the resultant force because force is a vector.
Since the image is not shown, let me assume that the two forces F1 nd F2 are in opposite directions. Hence; Fnet = 449 N - 390 N = 59 N
Now, the acceleration is obtained from; Fnet = ma
m = 3500 kg
a = Fnet/m = 59 N/3500 kg = 0.017 ms-2
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