Question 144223
Let's solve this with words at first:
y is the same as 3x.
Thus, x plus y must be x+3x. This is 4x. Thus, 4x must be 8. The value for x that makes this true is 2. Moreover, y must now be 3*2=6.

In symbols:

y=3x
x+y=8

-->

x+3x=8
4x=8
x=2

y=3(2)=6.