Answer :
prime factors of 42: 2x7x3
prime factors of 126: 2x3x3x7
prime factors of 210: 7x3x5x2
now we multiply all the factors that they all have in common: 2x3x7 which is 42. So the highest common factor is 42
prime factors of 126: 2x3x3x7
prime factors of 210: 7x3x5x2
now we multiply all the factors that they all have in common: 2x3x7 which is 42. So the highest common factor is 42