2x + 5y = 145
x + y = 50

For this, you've got to double one of the equations, and this will then allow you to eliminate part of the equation.
As you've now got one part, you've got to substitute this in, and this gives you;


Once you've done both equations, you have to ensure that they're correct, and in this case, they are.
Therefore, 
Hope this helps :)