The second best way is to set your miter gauge to 45 degree and cut the angles that way. You should have an extended fence for support.
Start with a 12" wide board and make the diagonals using the miter gauge, NOT the rip fence of course.
there are 2 - 12" sides and the long side, the hypotenuse, will be 16.9"
Other methods would include a sliding miter saw OR a radial arm saw set to 45 degrees.
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