Hola !
Tu queres lá regla de cramer , entonces yo voy usa - la a la regla :
X + Y = 7
2X - Y = 2
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<img src="https://tex.z-dn.net/?f=%5Cleft%7C%5Cbegin%7Barray%7D%7Bcc%7D1%261%5C%5C2%26-1%5Cend%7Barray%7D%5Cright%5Cleft%7C%5Cbegin%7Barray%7D%7Bcc%7D%26%26%5C%5C%26%26%5Cend%7Barray%7D%5Cright%20" />
∆ = - 1 - 2 = - 3
<img src="https://tex.z-dn.net/?f=%5Cleft%7C%5Cbegin%7Barray%7D%7Bcc%7D7%261%5C%5C2%26-1%5Cend%7Barray%7D%5Cright%5Cleft%7C%5Cbegin%7Barray%7D%7Bcc%7D%26%26%5C%5C%26%26%5Cend%7Barray%7D%5Cright%20" />
∆x = - 7 - 2 = - 9
<img src="https://tex.z-dn.net/?f=%5Cleft%7C%5Cbegin%7Barray%7D%7Bcc%7D1%267%5C%5C2%262%5Cend%7Barray%7D%5Cright%5Cleft%7C%5Cbegin%7Barray%7D%7Bcc%7D%26%26%5C%5C%26%26%5Cend%7Barray%7D%5Cright%20" />
∆y = 2 - 14 = - 12
X = ∆x / ∆ = - 9 / - 3 = 3
Y = ∆y / ∆ = - 12 / - 3 = 4
La respuesta es :
S = {3 , 4}.