Question 1108540
2y  is already in simplest form 
2    is already in simplest form
(x-1)-8x  can be simplified by combining the "x" with the "-8x"  to get -7x.  The result is -7x-1.  This is called "combining like-terms."   
y   is already in simplest form
2+1 can be simplified to  3  by adding the two numbers
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Simplification just means manipulating the expression to get it to its simplest form.

Do note that any higher-power terms in x  {{{ x^2 }}}, {{{ x^3 }}}, etc.  are simplified separately from the {{{x=x^1}}} and  constant {{{ x^0 }}} terms.   

For example {{{ (3x^3 + 2x^2 + x + 7) - (x^3 + x^2 + x + 5) }}}  simplifies (step by step) as follows:
 = {{{ (3x^3 +2x^2  + x + 7) - x^3 - x^2 - x - 5 }}}   <<< changed sign of 2nd paren's contents (multiplied the contents of the 2nd paren's by -1 and removed the parens. )

=  {{{ (3-1)x^3 + (2-1)x^2 + (1-1)x + (7-5) }}}   <<< just to show you how the coefficients add together. Normally you will do this math in your head, usually starting with the highest power of x and working your way down.
=  {{{  2x^3 + x^2 + 2 }}}        <<< notice there is no {{{ x^1 }}} because we ended up with 0*x