y = x + 4 [1]
3x + y = 16 [2]
You can take equation [1] and substitute it into equation [2], which will give you:
3x + y = 16
3x + (x + 4) = 16
4x + 4 = 16
You can now subtract 4 from both sides of the equation:
4x + 4 - 4 = 16 - 4
4x = 12
And now you can divide both sides of the equation by 4:
4x/4 = 12/4
x = 3
You can now substitute in this value into either equation [1] or [2]. I will use [1]:
y = x + 4
y = 3 + 4
y = 7
So the final solution is:
x = 3 and y = 7