1) Find the Highest common factor (HCF) of 28 and 63 by expressing them as prime factors.
28=4*7=2*2*7
63=9*7=3*3*7
The number that occurs in both is the HCF (if the same number appears twice or more in both put it to the power of x where x is the number of occurrences)
In this we can see that the HCF is 7, therefore you can divide both the numbers by 7.
28/63 ----> 4/9
4=2*2
9=3*3
There are no matching numbers for 4 and 9 therefore that is the fraction in its simplest form.