x + y = 4

y= 3x

--> Solve for x first so plug in 3x for y

so your new equation should look like this:

x + 3x = 4 (combine like terms by adding the x)

4x = 4 (now divide 4 to both sides)

x= 4  

Now to check for Y you have to plug in what you got for solving x which is x=4

y= 3x

--> y= 3(1)

y= 3          always remember to write down the answer like (x,y)

so your answer is (1,3)