If it is a right triangle then you can use the pythagorean theorem and you will get the hypotenuse which is equal to 13.93

If it is not a right triangle, assuming that the height(8) passes through the midpoint of the base(11.4/2 = 5.7), you have two right triangles with a height(8) and a base(5.7)

Using the pythagorean theorem, the hypotenuse of those two right triangles (which is the side of the isosceles triangle) is 9.82.