Question

{3x+7y=15; {x-3y=-3.

Answer 1

$3x + 7y = 15 \\ x - 3y = - 3 \\ \\ 3x + 7y = 15 \\ 3x - 9y = - 9 \\ \\ + 7y - ( - 9y) = 15 - ( - 9) \\ x = - 3 + 3y \\ \\ 16y = 24 \\ x = 3y - 3 \\ \\ y = 1.5 \\ x = 4.5 - 3 = 1.5$
Ответ: (1.5; 1.5)

Answer 2

$\left \{ {{3x+7y=15} \atop {3x-9y=-9}} \right.$ (1)

$\left \{ {{9x+21y=45} \atop {7x-21y=-21}} \right.$ (2)

из (1): $16y=24, y=1.5$

из (2): $16x=24, x=1.5$

Ответ: (1.5; 1.5)