again another Pythagorean Theorem i need help on, please and thank you

You commented that part of the instruction included "If the theorem cannot be applied, explain the reason."
The theorem cannot be applied. Since this is not a right triangle, we cannot solve using the Pythagorean Theorem unless we are given a height. If we have the height of this triangle, then we can break it into two different right triangles and solve.
What is a right triangle? A right triangle is a triangle that has an angle measurement of 90 degrees.
In summary, this is an isosceles triangle and the Pythagorean Theorem only applies to right triangles.