Question

How do I solve this system using substitution: y = x + 4; y = 3x

Answer 1

Y = x + 4
y = 3x

 3x = x + 4
- x  - x       
 2x = 4
  2     2
   x = 2
y = x + 4
y = 2 + 4
y = 6
(x, y) = (2, 6)

Answer 2

You just do 
3x=x+4
subtract x from both sides
2x=4
divide 2 from both sides
x=2
y=2+4
y=6
(x,y)= (2,6)