Question 38460
First step, plug x into the values of x
{{{x= 2y-1}}}

{{{2(2y-1)+y=81}}}
*[invoke explain_simplification "2(2y-1)+y=81" ]

y={{{highlight(16.6)}}}

Plug y into the equation to find x

{{{x= 2y-1}}}
{{{x=2(16.6)-1}}}
*[invoke explain_simplification "x=2(16.6)-1" ]

x={{{highlight(32.2)}}}
If you need more detailed explanations, you can see the lesson I wrote on substitution.
Hope this helps!