`(5)/(6y+3)-(a)/(8y+4)` `=5/(3(2y+1))-a/(4(2y+1))`

`=20/(12(2y+1))-(3a)/(12(2y+1))`

`=(20-3a)/(12(2y+1))`

We have factored out the 3 at the bottom of the first fraction and the 4 at the bottom of the second fraction.

In the 3rd line, we find the lowest common denominator, 12(2y + 1), and multiply top and bottom of the two fractions accordingly.

The last line is a tidy up step.

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