Alias frequency Formula
Suppose that the sampling is done at a rate of $1000$ Hz, one sample every millisecond. Suppose also that the signal being sampled is at $3200$ Hz,
the first sample is at the peak of the sinusoid. The next sample,
will be taken one millisecond later during which time the sinusoid will 
have gone through $3.2$ periods, and so the next sample will have the 
same value as if the sinusoid had gone through $0.2$ periods, not $3.2$ 
periods. The one after that will be $0.4$ periods away from the peak, and
so on.  This is exactly the same set of samples that we would gave gotten
if we had been sampling a $200$ Hz sinusoid.  In one millisecond it would have
progressed through $0.2$ of its period of $5$ milliseconds and so on.
In other word, just by looking at the samples alone we cannot tell whether
the samples came from a $3200$ HZ signal or from a $200$ Hz signal.

If the signal being sampled was at $2800$ Hz, then we would get samples
corresponding to $0$, $-0.2$ of the period, $-0.4$ of the period and
so on.  But because sinusoids look the same in either direction in time,
these samples also look like they are the result of sampling a $200$
Hz signal.  This is the reason why the formula you are given, viz.


  Aliased frequency is the absolute difference between the actual signal frequency
  and the nearest integer multiple of the sampling frequency.


works to give you the right answer.