1. 
x + 2y = 4,    (1)
x = y - 5.     (2)


Hey, you are lucky! They just did almost everything for you.

They just expressed x via y in the equation(2), and all you need to do is to substitute it instead of x into equation (1).

In this way you will get a single equation for "y" with the only unknown "y":

(y-5) + 2y = 4.    (3)

You can easily solve it for y. 
When you complete it for "y", calculate "x" via (2).

2.
y = x-1,             (1)   
y = .    (2)

Take the expression (1) for "y" and substitute it into (2). You will get

x-1 = ,

which is a single equation on "x".

You are lucky for the second time!