Area of square is length x breadth where both length and breadth are the same. So using timetables,

1cm x 1cm = 1cm square
2cm x 2cm = 4cm square

And so on.

So for 36cm square and 81cm square, it is 6cm and 9cm respectively.

Therefore, you will have height of triangle = 6cm, length of triangle = 9cm.

Triangle area is 1/2 (base x height), so you will have
1/2 x 6cm x 9cm = 27cm square