The answer is 8. Since BD is only 1/2 of the line and it equals 4. Since the other half of the line is the same length as BD the total would be 8 because 4•2=8.
Since this isosceles triangle has been broken down into two right triangles, and we already know that two sides are the exact same (note that there are 2 x's) that means BD and BC are going to be the exact same length. Since BD is 4, that means BC will be 4, and that means