We simply need to use the Pythagorean theorem:
a^2 + b^2 = c^2 (c^2 is the hypertenuse)
The equation will look like this:
14^2 + 14^2 = x^2
196 + 196 = x^2
x^2 = 392
So the answer is the square root of 392:
14
If you want a decimal form:
14 * 1.414 = 19.796