Question

seja y=2x+18 e y=-x+30

Answer 1

- 2x + y = 18
   x + y = 30     x(-1)

-2x + y = 18
-x  - y = - 30
-3x = - 12          x( -1) 
 3x = 12
 x = 12/3
x = 4

y = 2 . 4 + 18
y = 8 + 18
y = 26

Answer 2

y=2x+18
y=-x+30
   (y)   = (-x+30)
2x+18 =  -x+30
2x+x = 30-18
3x = 12
x = 12/3
x = 4
substituir x na equaçao:
y = -(4) +30
y = 30 - 4
y = 26