How to add the 7 + 3 together

Hi, does anyone know how to add the 7 and 3 together in the book with the pencil. My pencil has disappeared after adding a square to the left hand side of the page. Hoping someone can offer advice. Thanks

You need to use the pencil on the 7 and the 3 to get 10. Then erase the Tic Tac Toe grids to get the square.

I got the square, how do you get the seven and three added together? Ive tried and tried and tried. so frustrated!!!

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