Yeah, x= 7/2 and y= -5/2
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Answer
Find out the what fraction of the sandcastle did they build together .
To prove
Let us assume that the fraction of the sandcastle they build together be x .
As given


Than the equation becomes

L.C .M of (3,7) = 21
solving


Therefore the fraction of the sandcastle did they build together be

Hence proved
Combine like terms... 2s and 4s are like terms so that's 6s 4,2,3 are like terms add that up its 9. So it would look like this 6s+9