If a number is the square of a prime number, it has
exactly 3 divisors. Give an example and explain.

Take the prime number 2.  Square it.  Get 4.  4
has exactly three divisors. They are 1, 2 and 4,
because all three of those but no others will divide
evenly into 4.

Take the prime number 3.  Square it.  Get 9.  9 has 
exactly three divisors. They are 1, 3 and 9, because 
all three of those but no others will divide evenly into 9. 

Take the prime number 5.  Square it.  Get 25.  25 has
exactly three divisors. They are 1, 5 and 25, because all 
three of those but no others will divide evenly into 25.

Take the prime number 11.  Square it.  Get 121.  121 
has exactly three divisors. They are 1, 11 and 121, because 
all three of those but no others will divide evenly into 11.

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Now, to elaborate,  you might wonder if that would work if 
you started with a number other than a prime number.  The 
answer is no, but we need to demonstrate this:

Take the non-prime number 4.  Square it.  Get 16.  But 16 
has FIVE divisors. They are 1,2,4,8,16. because all FIVE of 
those but no others will divide evenly into 16.

So it doesn't work for square of the non-prime 4.

Take the non-prime number 1.  Square it.  Get 1.  But 1 
has only ONE divisor. That is 1 itself. because that is the 
only number that will divide evenly into 1.

So it doesn't work for the square of the non-prime 1 either.

Edwin