Question 973010
<pre>
             3 years ago         today          1 year future

Evan           y-3                 y            y+1

Sister         3(y-3)=x-3          x              2(y+1)=x+1
</pre>
{{{system(3(y-3)=x-3,2(y+1)=x+1)}}}


{{{3y-9=x-3}}}
{{{3y-x=9-3}}}
{{{3y-x=6}}} from first equation


{{{2y+2=x+1}}}
{{{2y-x=1-2}}}
{{{2y-x=-1}}} from second equation


Subtracting,
{{{highlight(y=7)}}}, Evan's age today

{{{3y-6=x}}}
{{{x=21-6}}}
{{{highlight(x=15)}}}, his sister's age today