First we need to recall what we know. We know that an equilateral triangle has 3 sides of equal length and 3 angles that are equal. Since ALL the angles in a triangle add up to 180º then 180 divided by 3 must be 60º.
and since we know the bottom of the triangle is 8 that means x must be 4 in order to have all sides equal. This is assuming that is an quadrilateral triangle. Sorry its hard to make out.
Answer:
x=4
I Hope this helped :D