Is it possible, for some choice of positive whole numbers m and n, that 63m = 21n ? justify your answer?

Yes, as long as the value of n is three times as big as the value of m. 

We can determine this by dividing the coefficient of n away. 

63m = 21n ---> divide both by 21. 
3m = n

To satisfy this, we could use the simple m = 1 and n = 3, which obviously makes the statement true and satisfies our positive whole numbers criteria. We could also use something as large as m = 121 and n = 363.