Here's another way to do it. If you draw the perpendicular from the vertex where the two equal sides meet to the opposite side, you get two congruent right triangles. The hypotenuse of each is one of the two equal sides and so has length 2. One of the legs is half the other side and so has length x/2. The other leg is the altitude of the original triangle and, taking its length to be "H", by the Pythagorean theorem,

. Then

so

. That's exactly what you had!

Now, the area of a triangle is (1/2)*height*base which, here, is

.

When you wrote "A = 1/2 X (x^2-1/4(2)^2)", first, it is not clear if "X" and "x" are the same or if you mean "X" as "multiplication". The crucial problem is that you have confused "a" and "b". Here, a= x while b= 2, not the other way around. That's the problem with just memorizing formulas. You have to be careful that you are assigning the correct values to the parameters.