Solve the equation for x, it means express x in terms of y
[tex]\begin{gathered} 3(x+2y)=2x+5y \\ 3x+6y=2x+5y \\ 3x-2x=5y-6y \\ x=-y \end{gathered}[/tex]Once found the expression of x in terms of y, replace for the given value of y to find x
[tex]\begin{gathered} y=5 \\ x=-y \\ x=-5 \end{gathered}[/tex]