Why do we not use a leg-leg (ll) theorem that says if you know that the legs of two right triangles are congruent, then the triangles are congruent?

I am sorry I never hear of the LL Thereom, but I can tell you that there are 3 ways to prove that two triangles are congruent known as ASA,,SSS,Sas, A standing for Angle and S standing for side

Step-by-step explanation:

An isosceles triangle has two equal sides. It follows from this, that the angles at the bases must be the same, and also that the third angle must equal the sum of these two equal angles from 180 degrees, since there is 180 degrees in a triangle.

But what about congruency: when two triangles are the same, they will have exactly the same three sides and exactly the same three angles, that is what is meant by congruence.

To give you a similar example in 2-Dimensions. if 2 parallelograms have the same lengths of their sides and also the same angles we say they are "equal". (although one could be slanted to the right, and one could be slanted to the left).  

a = 11 : 4

b = 38

c = 1 for every 6

the answer to the expression is 83