The diagonal cuts the square into two congruent triangles. Since it's a square, one of the corners will have a 90 degree angle, and the other two will have 45 degree angles since the triangle was cut out of a square. With the angle measurements known, you can use trigonometric functions in order to find the other sides. Ex: sin(45)=x/9 now you can solve x to find the missing side. Since it is a square, all other sides will have the same measurement.