y=3, x+y=5

Since y=3 is given, that means that anywhere you see y, 
you can put 3 in place of it.

So put 3 in place of y in this:

x+y=5

and you get

x+3=5

And you can see that 2 is the only number x could be. So

y=3 and x=2
-------------------
You can also solve

x+3=5

by adding -3 to both sides:

x + 3 = 5
   -3  -3
---------
x     = 2

Edwin