Question 981528
x and y the side lengths.
x=y-3, as described.

Initial area, xy.


Next part of description,
{{{(x+1)(y+1)=xy+18}}}
{{{xy+x+y+1=xy+18}}}
{{{x+y+1=18}}}
substitute for x from the description,
{{{(y-3)+y+1=18}}}
{{{2y-3+1=18}}}
..
{{{highlight(y=10)}}}
and then
{{{highlight(x=7)}}}.