16. It's just area of one of the triangles multiplied by 4:
17. There are 5 3*3 squares in the figure so the entire area is:
18. The height of the weird triangle at the bottom right is 35-24 = 11 and the base is 42-33 = 9. The width of the rectangle that remains is then 33 and the length is 35. Let's add all these areas up:
19. The area of the garden is:

From this we subtract the area of the tree which is just a square of side length 7 (the ? = 7):

The total area of garden (your answer):

Hope this helps!