Answer :

7x=20+3y
X=(20+3y)/7

Y=5x-4
Y=5((20+3y)/7)-4
Y=((100+15y)/7)-4
7y=100+15y-28
7y=72+15y
7y-15y=72
-8y=72
Y=-9

7x-3y=20
7x-3(-9)=20
7x+27=20
7x=20-27
7x=-7
X=-1

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