6y - 3g + 2y - 4g

Rearrange the terms to get the ones with y together and the ones with
g together

6y + 2y - 3g - 4g

Factor y out of the first two terms and -g out of the last two:

 y(6+2) - g(3+4)

Add 6+2 and get 8.  Add 3+4 and get 7

   y(8) - g(7)

Put the number left of the letter using the commutative principle

     8y - 7g

The others are done the same way.

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Edwin