The correct answer is 76.
There are 9 points in the grid. You can pick any 3 out of these 9 points in 9*8*7/(3*2*1) = 84 ways.
However,
3 will form horizontal lines in the grid,
3 will form vertical lines in the grid, and
2 will form diagonal lines in the grid.
None of these 8 triplets will form triangles, but all the rest will.
So the answer is 84-8=76.
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A 3x3 grid is made up of 9 small squares. However there are also squares of larger sizes. There are 4 2x2 squares. There is also the one big square that uses all the 3x3 area. In total this gives us 9+4+1 = 14. Thus there are 14 squares in a 3x3 grid.
You really should do your own homework - this is a question designed to make you analyse number patterns and devise a method to predict the answer that can be applied to grids of differing size. If we start with a square cut into a 3x3 grid, we can count the nine single (1x1) squares in the grid, the one 3x3 square, and then four 2x2* squares, making a total of 14. Try it out, then work your way up to 6x6 (a 36 square grid) by way of 4x4 and 5x5, looking to see how the grid's dimensions correlate to the number of varying-sized squares that can be counted. As a tip- in a 6x6 grid, you will have one 6x6 square, thirty-six 1x1 squares, and how many 2x2, 3x3, 4x4, and 5x5 squares? *The squares can overlap, obviously.
Multiple them! 3x3=9 squares!
3x3 = 9 square feet.