Ok, what is a common thing you know about squares? All 4 sides are equal and the area of a square is a multiplication of
One side x one side
So, what same number multiplied by that same number gives you 81 and 49?
It would be:
9x9=81
&
7x7=49
Now you have found the base and height of your triangle.
The area of a triangle is 1/2 x base x height.
So,
1/2 x 7 x 9 =31.5 and you would get your answer of the area of the triangle