Question 1094731
You have {{{x=-1/2}}} .
Substituting, you get
{{{3(-1/2)-5y=7}}}
Simplifying and solving,
{{{-3/2-5y=7}}}
{{{-3/2-5y-7+5y=7-7+5y}}}
{{{-3/2-7=5y}}}
{{{-3/2-14/2=5y}}}
{{{-17/2=5y}}}
{{{(1/5)*(-17/2)=(1/5)*5*y}}}
{{{-17/10=1*y}}} .
So, {{{y=highlight(-17/10=-1.7)}}} is the answer.
{{{-17/10}}} goes better with {{{1/2}}} , both as fractions,
but if you are allergic to fractions,
you could have started with {{{x=0.5}}} and ended with {{{y=-1.7}}} .
I have no idea where you got a 13 in the denominator.
You may be trying to follow a misremembered procedure,
but memorizing calculation procedures is the wrong way to learn math.
It is better to understand what the procedure means.