Question

SISTEMA
x-y=21
x-5=2y

Answer 1

$\left \{ {{x\,-\,y\,=\,21} \atop {x\,-\,5\,=\,2y}} \right. \Leftrightarrow \left \{ {{x\,-\,y\,=\,21} \atop {x\,-\,2y\,=\,5}} \right. \\ \\ \Rightarrow -y -(-2y) = 21 - 5 \Leftrightarrow -y + 2y = 16 \Leftrightarrow \framebox {y = 16} \\ \\ \Rightarrow x - y = 21 \Leftrightarrow x - 16 = 21 \Leftrightarrow \framebox {x = 37}$

Answer 2

x-y=21   ==>x=21+y
x-5=2y   ==>21+y-5=2y ==> 21-5=2y-y ==>16=y ==>y=16

x=21+y =21+16=37