Me !me measure the diagonal of a triangle. so stressed. all i can tell u is that one side is 11 inches and the other 2 are 8 inches. it has a right angle. !

Sounds as tho' you have an isosceles triangle (a triangle with 2 equal sides).  If this triangle is also a right triangle (with one 90-degree angle), then the side lengths MUST satisfy the Pythagorean Theorem.

Let's see whether they do.

8^2 + 8^2 = 11^2  ???
 64    + 64 = 121?  NO.  This is not a right triangle.

If you really do have 2 sides that are both of length 8, and you really do have a right triangle, then:

8^2 + 8^2 = d^2, where d=hypotenuse.  Then 64+64 = d^2, and 

d = sqrt(128) = sqrt(8*16) = 4sqrt(8) = 4*2*sqrt(2) = 8sqrt(2) = 11.3.

11 is close to 11.3, but still, this triangle cannot really have 2 sides of length 8 and one side of length 11.