.
    Y = 10X + 2     (1)
    2X + Y = 26     (2)
From equation(1), you just have an expression for Y via X.
Substitute this expression into equation (2). You will get then
    2X + (10x + 2) = 26.
Simplify and solve for X
    12X = 26 - 2
    12X = 24
      X = 24/12 = 2.
You just found X:  X = 2.
To find Y, substitute this value X= 2 into equation (2).  You will get
    Y = 10*2 +2 = 22.
Now you have everything to find the difference X-Y.
I leave this last step to you.
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To the tutor @richwmuller :
    I agree with you: for many visitors it is the way to get a solution and/or explanation that they practically 
    will get NEVER and from NOWHERE.
    I do it usually 10 - 20 times per day, when I find the problem formulation correct (or close to be correct)
    and then I think that my participation will be useful.
    Have a nice day (!)