Question

Solve 3x+2y=7 and x-4y=-21 by using substitution

Answer 1

$\left \{ {{3x+2y=7\ \ | *2} \atop {x-4y=-21}} \right. \\\\ \left \{ {{6x+4y=14} \atop {x-4y=-21}} \right.\\\\+----\\Addition\\\\ 7x=14-21\\ 7x= -7\ \ \ | divide\ by\ 7\\\\ x=-1\\ 2y=7-3x\\ y=\frac{7-3x}{2}=\frac{7+3}{2}=5\\\\Solution\ \\ \left \{ {{y=5} \atop {x=-1}} \right.$

Answer 2

3x + 2y = 7
x - 4y = - 21

6x + 4y = 14
x - 4y = - 21

6x + x = 14 - 21
7x = - 7
x = - 7 /7
x = - 1

y = (7 - 3x)/2 
y = (7 - 3 x (- 1))/2
y = (7 - (- 3))/2
y = (7 + 3)/2
y = 10/2
y = 5

Ans : x = - 1
        y = 5