if you want to find the common denominator you can just multiply the denominators of the given fractions which would yield 24 thus:
but in this case a smaller common denominator can be found which is 12:
the same result could be obtained in the first method if we canel the highest common factor of 26 and 24 which is 2.
the lowest common denominator is 12 the denominator of 5x is 6 thus it has to be multiplied by 2 so that it will be 12 and so the nominator has to be also multiplied by 2, this is where 10x comes from. The denominator of the second fraction is 4 thus it has to be multiplied by 3 to get 12 and the nominator also has to be multiplied by by 3, this is where 3x comes from.