There is enough info. Look at it as 2 right triangles plus a rectangle in the middle. Find height by using Pythagoras's theorem. At 81 I don't recall the math.
Yet another way to solve it. Now it is all Pathagorean.View attachment 465621
It should be 24x24 - 10x10
Sorry I did not get in on this in time to work this one. I love working these. a^2 +b^2=c^2 or in words, the square of the hypotenuse equals the sum of the squares of the other two sides. Must be a right triangle of course. Once a nerd, always a nerd I guess.