Answer:
I'm guessing you are talking about an isosceles RIGHT triangle.
In such a triangle, the length of the hypotenuse equals the length of either leg times the square root of 2.
hypotenuse = 14 * square root of 2 =
14 * 1.4142135623731 = 19.79898987322330
or rounding that equals 19.80 inches
Explanation: