Question

3 x - y = 18 x mais y = 10

Answer 1

3x-y=18
x+y=10 Corta o y e -y.

4x=28
  x=28
      4
x=7

7+y=10
y=10-7
y=3


Testar.....

7+3=10
3.7-3=
21-3=18

Answer 2

         3x -y=18
           x- y =10
        _________
         4x     = 28
4x = 28
x= 28/4
x= 7

3.7- y=18
21- = 18
-y = = 21 + 18   
-y= -3   (multiplica por -1)
y= 3

S= (3.7}