To learn how to simplify fractions, try here.
I assume that you have heard about prime decomposition (?): Each natural number can be written as a product of prime factors OR (if the number is a prime factor itself) as .
The first prime numbers (which you should learn by heart) are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, ...
There are an infinite number of prime numbers.
I examined 21: It is divisible by 3 that means 3 must be a factor in 21. The remainder is 7 which is a prime factor too. Thus
Do the same with 35.